Liquid crystal display device and method for driving the same

ABSTRACT

To increase the frequency of input of image signals in terms of design in a field-sequential liquid crystal display device. Image signals are concurrently supplied to pixels provided in a plurality of rows among pixels arranged in matrix in a pixel portion of the liquid crystal display device. Thus, the frequency of input of an image signal to each pixel can be increased without change in response speed of a transistor or the like included in the liquid crystal display device.

TECHNICAL FIELD

The present invention relates to a liquid crystal display device and amethod for driving the liquid crystal display device. In particular, thepresent invention relates to a liquid crystal display device in whichimages are displayed by a field sequential method, and a method fordriving the liquid crystal display device.

BACKGROUND ART

A color filter method and a field sequential method are known as displaymethods for liquid crystal display devices. In a liquid crystal displaydevice in which images are displayed by a color filter method, aplurality of subpixels each having a color filter that transmits onlylight with a wavelength of a given color (e.g., red (R), green (G), orblue (B)) are provided in each pixel. A desired color is produced insuch a manner that transmission of white light is controlled in eachsubpixel and a plurality of colors are mixed in each pixel. On the otherhand, in a liquid crystal display device in which images are displayedby a field sequential method, a plurality of light sources that emitlights of different colors (e.g., red (R), green (G), and blue (B)) areprovided. A desired color is produced in such a manner that theplurality of light sources sequentially emit light and transmission oflight of each color is controlled in each pixel. In other words, adesired color is produced by dividing the area of one pixel betweenlights of given colors in a color filter method, whereas a desired coloris produced by dividing a display period between lights of given colorsin a field sequential method.

The field-sequential liquid crystal display device has the followingadvantages over the color-filter liquid crystal display device. First,in the field-sequential liquid crystal display device, it is notnecessary to provide subpixels in a pixel. Thus, the aperture ratio orthe number of pixels can be increased. In addition, in thefield-sequential liquid crystal display device, it is not necessary toprovide a color filter. That is, loss of light due to light absorptionin the color filter does not occur. For that reason, the transmittancecan be increased and power consumption can be reduced.

Patent Document 1 discloses a field-sequential liquid crystal displaydevice. Specifically, Patent Document 1 discloses a liquid crystaldisplay device that includes pixels each including a transistor forcontrolling input of an image signal, a signal storage capacitor forholding the image signal, and a transistor for controlling transfer ofelectric charge from the signal storage capacitor to a display pixelcapacitor. In the liquid crystal display device having this structure,writing of an image signal to the signal storage capacitor and displaycorresponding to electric charge held at the display pixel capacitor canbe performed concurrently.

REFERENCE

Patent Document 1: Japanese Published Patent Application No. 2009-042405

DISCLOSURE OF INVENTION

In a field-sequential liquid crystal display device, the frequency ofinput of an image signal to each pixel needs to be increased. Forexample, in the case where images are displayed by a field sequentialmethod in a liquid crystal display device including three kinds of lightsources each emitting one of red (R) light, green (G) light, and blue(B) light, the frequency of input of an image signal to each pixel needsto be at least three times that of a color-filter liquid crystal displaydevice. Specifically, in the case where the frame frequency is 60 Hz, animage signal needs to be input to each pixel 60 times per second in thecolor-filter liquid crystal display device; whereas an image signalneeds to be input to each pixel 180 times per second in the case whereimages are displayed by a field sequential method in the liquid crystaldisplay device including the three kinds of light sources.

Note that high-speed response of an element included in each pixel isrequired, accompanied by the increase in the input frequency of imagesignals. Specifically, the increase in mobility of a transistor providedin each pixel is required, for example. However, it is not easy toimprove characteristics of the transistor or the like.

In view of the above, an object of one embodiment of the presentinvention is to increase the frequency of input of image signals.

The object can be achieved in such a manner that image signals areconcurrently supplied to pixels provided in a plurality of rows amongpixels arranged in matrix in a pixel portion of a liquid crystal displaydevice.

In other words, one embodiment of the present invention is a liquidcrystal display device which includes a first signal line, a secondsignal line, a third signal line, a first pixel, a second pixel, and athird pixel. The first pixel comprises a first transistor, a secondtransistor, a third transistor, and a first liquid crystal element. Agate of the first transistor is electrically connected to a first scanline, and one of a source and a drain of the first transistor iselectrically connected to the first signal line. A gate of the secondtransistor is electrically connected to a second scan line, and one of asource and a drain of the second transistor is electrically connected tothe second signal line. A gate of the third transistor is electricallyconnected to a third scan line, and one of a source and a drain of thethird transistor is electrically connected to the third signal line. Oneof electrodes of the first liquid crystal element is electricallyconnected to the other of the source and the drain of the firsttransistor, the other of the source and the drain of the secondtransistor, and the other of the source and the drain of the thirdtransistor. The second pixel comprises a fourth transistor, a fifthtransistor, a sixth transistor, and a second liquid crystal element. Agate of the fourth transistor is electrically connected to a fourth scanline, and one of a source and a drain of the fourth transistor iselectrically connected to the first signal line. A gate of the fifthtransistor is electrically connected to a fifth scan line, and one of asource and a drain of the fifth transistor is electrically connected tothe second signal line. A gate of the sixth transistor is electricallyconnected to a sixth scan line, and one of a source and a drain of thesixth transistor is electrically connected to the third signal line. Oneof electrodes of the second liquid crystal element is electricallyconnected to the other of the source and the drain of the fourthtransistor, the other of the source and the drain of the fifthtransistor, and the other of the source and the drain of the sixthtransistor. The third pixel comprises a seventh transistor, an eighthtransistor, a ninth transistor, and a third liquid crystal element. Agate of the seventh transistor is electrically connected to a seventhscan line, and one of a source and a drain of the seventh transistor iselectrically connected to the first signal line. A gate of the eighthtransistor is electrically connected to an eighth scan line, and one ofa source and a drain of the eighth transistor is electrically connectedto the second signal line. A gate of the ninth transistor iselectrically connected to a ninth scan line, and one of a source and adrain of the ninth transistor is electrically connected to the thirdsignal line. One of electrodes of the third liquid crystal element iselectrically connected to the other of the source and the drain of theseventh transistor, the other of the source and the drain of the eighthtransistor, and the other of the source and the drain of the ninthtransistor. Further, from the first signal line, a first image signal issupplied in a first horizontal scan period included in a first samplingperiod, a second image signal is supplied in a first horizontal scanperiod included in a second sampling period, and a third image signal issupplied in a first horizontal scan period included in a third samplingperiod. From the second signal line, a fourth image signal is suppliedin the first horizontal scan period included in the first samplingperiod, a fifth image signal is supplied in the first horizontal scanperiod included in the second sampling period, and a sixth image signalis supplied in the first horizontal scan period included in the thirdsampling period. From the third signal line, a seventh image signal issupplied in the first horizontal scan period included in the firstsampling period, an eighth image signal is supplied in the firsthorizontal scan period included in the second sampling period, and aninth image signal is supplied in the first horizontal scan periodincluded in the third sampling period. Additionally, in the firsthorizontal scan period included in the first sampling period, aselection signal is supplied to the first scan line, the fifth scanline, and the ninth scan line, and a non-selection signal is supplied tothe second scan line, the third scan line, the fourth scan line, thesixth scan line, the seventh scan line, and the eighth scan line. In thefirst horizontal scan period included in the second sampling period, aselection signal is supplied to the third scan line, the fourth scanline, and the eighth scan line, and a non-selection signal is suppliedto the first scan line, the second scan line, the fifth scan line, thesixth scan line, the seventh scan line, and the ninth scan line. In thefirst horizontal scan period included in the third sampling period, aselection signal is supplied to the second scan line, the sixth scanline, and the seventh scan line, and a non-selection signal is suppliedto the first scan line, the third scan line, the fourth scan line, thefifth scan line, the eighth scan line, and the ninth scan line. Notethat the first scan line, the fourth scan line, and the seventh scanline are electrically connected to a first shift register, and theselection signal and the non-selection signal are supplied from thefirst shift register. The second scan line, the fifth scan line, and theeighth scan line are electrically connected to a second shift register,and the selection signal and the non-selection signal are supplied fromthe second shift register. The third scan line, the sixth scan line, andthe ninth scan line are electrically connected to a third shiftregister, and the selection signal and the non-selection signal aresupplied from the third shift register.

According to one embodiment of the present invention, a liquid crystaldisplay device includes a first signal line supplied with a first imagesignal in a first horizontal scan period in a first sampling period, andsupplied with a second image signal in a second horizontal scan periodin a second sampling period; a second signal line supplied with a thirdimage signal in the first horizontal scan period, and supplied with afourth image signal in the second horizontal scan period; a first scanline and a second scan line supplied with a selection signal in thefirst horizontal scan period, and supplied with a non-selection signalin the second horizontal scan period; a third scan line and a fourthscan line supplied with a non-selection signal in the first horizontalscan period, and supplied with a selection signal in the secondhorizontal scan period; a first pixel electrically connected to thefirst signal line, the second signal line, the first scan line, and thethird scan line, supplied with the first image signal in the firsthorizontal scan period, and supplied with the fourth image signal in thesecond horizontal scan period; a second pixel electrically connected tothe first signal line, the second signal line, the second scan line, andthe fourth scan line, supplied with the third image signal in the firsthorizontal scan period, and supplied with the second image signal in thesecond horizontal scan period; a first shift register configured tosupply the selection signal to the first scan line in the firsthorizontal scan period and supply the selection signal to the fourthscan line in the second horizontal scan period; and a second shiftregister configured to supply the selection signal to the second scanline in the first horizontal scan period and supply the selection signalto the third scan line in the second horizontal scan period.

According to another embodiment of the present invention, a liquidcrystal display device includes a first signal line supplied with afirst image signal in a first horizontal scan period in a first samplingperiod, and supplied with a second image signal in a second horizontalscan period in a second sampling period; a second signal line suppliedwith a third image signal in the first horizontal scan period, andsupplied with a fourth image signal in the second horizontal scanperiod; a first scan line and a second scan line supplied with aselection signal in the first horizontal scan period and the secondhorizontal scan period; a first pixel electrically connected to thefirst signal line and the first scan line, supplied with the first imagesignal in the first horizontal scan period, and supplied with the secondimage signal in the second horizontal scan period; a second pixelelectrically connected to the second signal line and the second scanline, supplied with the third image signal in the first horizontal scanperiod, and supplied with the fourth image signal in the secondhorizontal scan period; a first shift register configured to supply theselection signal to the first scan line in the first horizontal scanperiod and the second horizontal scan period; and a second shiftregister configured to supply the selection signal to the second scanline in the first horizontal scan period and the second horizontal scanperiod.

Note that in a liquid crystal display device, AC voltage is applied to aliquid crystal element provided in a pixel in order to preventdeterioration of liquid crystal (called burn-in). For that reason, atransistor used in a pixel portion preferably has a withstand voltagehigher than or equal to about ten and several volts. Moreover, theoff-state current of the transistor needs to be low in order to maintaina voltage held at the liquid crystal element. In contrast, for atransistor used in a signal line driver circuit, high-speed operation ismore important than high withstand voltage in order to keep high qualityof images displayed in the liquid crystal display device.

In view of the above, a transistor including a channel formation regionformed using an oxide semiconductor is used in a pixel portion in aliquid crystal display device according to one embodiment of the presentinvention. The bandgap of the oxide semiconductor is 3.0 eV to 3.5 eV,which is about three times that of silicon. The wide band gap of theoxide semiconductor is advantageous in increasing the withstand voltageof the transistor. In addition, an oxide semiconductor that is purifiedby reduction in concentration of impurities such as moisture orhydrogen, which serves as electron donors (donors), that is, a purifiedOS is an intrinsic semiconductor (an i-type semiconductor) or asubstantially intrinsic semiconductor. Therefore, a transistor includingthe purified oxide semiconductor has very low off-state current inaddition to high withstand voltage.

Furthermore, in the liquid crystal display device according to oneembodiment of the present invention, a transistor including acrystalline semiconductor such as polycrystalline or single crystalsilicon or polycrystalline or single crystal germanium is used in adriver circuit that needs to operate at higher speed than the pixelportion, such as a signal line driver circuit. The crystallinesemiconductor has higher mobility than the oxide semiconductor.Accordingly, a signal line driver circuit can operate at high speed byusing the crystalline semiconductor for a transistor.

By employing different semiconductors and different processes for acircuit that needs to operate at high speed and a circuit that needs tohave high withstand voltage as described above, semiconductor elementswith optimum structures can be separately formed in accordance withfeatures required for the circuits, without complication of theprocesses.

In a scan line driver circuit, a transistor including a crystallinesemiconductor such as a polycrystalline semiconductor or a singlecrystal semiconductor may be used as in the signal line driver circuit,or a transistor in which a channel formation region is formed using anoxide semiconductor may be used as in the pixel portion.

A transistor including a crystalline semiconductor such aspolycrystalline or single crystal silicon or polycrystalline or singlecrystal germanium can be formed using a silicon wafer, an SOI (siliconon insulator) substrate, a polycrystalline semiconductor film formed onan insulating surface, or the like.

An SOI substrate can be manufactured by, for example, an attachmentmethod such as UNIBOND (registered trademark) typified by Smart Cut(registered trademark), epitaxial layer transfer (ELTRAN, registeredtrademark), a dielectric separation method, or plasma assisted chemicaletching (PACE); or separation by implanted oxygen (SIMOX).

A semiconductor film of silicon deposited over a substrate having aninsulating surface may be crystallized by a known technique. Examples ofa known technique of crystallization are a laser crystallization methodusing a laser beam and a crystallization method using a catalyticelement. Alternatively, a crystallization method using a catalyticelement and a laser crystallization method may be used in combination.In the case where a substrate with high heat resistance, such as aquartz substrate, is used, it is possible to combine any of thefollowing crystallization methods: a thermal crystallization method withan electrically heated oven, a lamp anneal crystallization method withinfrared light, a crystallization method with a catalytic element, andhigh temperature annealing method at about 950° C.

In the liquid crystal display device according to one embodiment of thepresent invention, image signals can be concurrently supplied to pixelsplaced in a plurality of rows among pixels arranged in matrix. Thus, thefrequency of input of an image signal to each pixel can be increasedwithout change in response speed of a transistor or the like included inthe liquid crystal display device.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1A illustrates a structural example of a liquid crystal displaydevice, and

FIG. 1B illustrates a configuration example of a pixel;

FIG. 2 illustrates a structural example of a scan line driver circuit;

FIG. 3 illustrates output signals of shift registers;

FIG. 4A illustrates a structural example of a signal line drivercircuit, and FIG. 4B illustrates a structural example of a backlight;

FIG. 5 illustrates an operation example of a liquid crystal displaydevice;

FIG. 6A illustrates a structural example of a liquid crystal displaydevice, and FIGS. 6B to 6D each illustrate a configuration example of apixel;

FIG. 7A illustrates a structural example of a scan line driver circuit,and FIG. 7B illustrates output signals of shift registers;

FIG. 8 illustrates a structural example of a signal line driver circuit;

FIG. 9 is a cross-sectional view illustrating an example of atransistor;

FIG. 10 shows characteristics of a transistor;

FIG. 11 is a circuit diagram for evaluating characteristics of atransistor;

FIG. 12 is a timing chart for evaluating characteristics of atransistor;

FIG. 13 shows characteristics of a transistor;

FIG. 14 shows characteristics of a transistor;

FIG. 15 shows characteristics of a transistor;

FIGS. 16A to 16C are cross-sectional views each illustrating an exampleof a transistor;

FIG. 17 illustrates a specific example of a cross section of a pixel;

FIGS. 18A to 18C each illustrate a specific example of connectionbetween terminals;

FIGS. 19A to 19C are perspective views each illustrating a specificexample of a liquid crystal display device;

FIGS. 20A and 20B are a top view and a cross-sectional view illustratinga specific example of a liquid crystal display device;

FIG. 21 is a perspective view illustrating a specific example of aliquid crystal display device;

FIGS. 22A and 22B each illustrate a specific example of a touch panel;

FIGS. 23A and 23B illustrate a specific example of a touch panel;

FIGS. 24A to 24D are cross-sectional views illustrating a specificexample of a process for manufacturing a transistor; and

FIGS. 25A to 25F each illustrate an example of an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the accompanying drawings. Note that the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that a variety of changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beconstrued as being limited to the description of the embodiments.

Embodiment 1

In this embodiment, an example of a liquid crystal display device inwhich images are displayed by a field sequential method will bedescribed with reference to FIGS. 1A and 1B, FIG. 2, FIG. 3, FIGS. 4Aand 4B, and FIG. 5.

(Structural Example of Liquid Crystal Display Device)

FIG. 1A illustrates a structural example of a liquid crystal displaydevice. The liquid crystal display device in FIG. 1A includes a pixelportion 10; a scan line driver circuit 11; a signal line driver circuit12; 3n scan lines 131, 3n scan lines 132, and 3n scan lines 133 (n is anatural number of 2 or more) arranged parallel or approximately parallelto each other; and m signal lines 141, m signal lines 142, and m signallines 143 (m is a natural number of 2 or more) arranged parallel orapproximately parallel to each other. The potentials of the scan lines131, 132, and 133 are controlled by the scan line driver circuit 11. Thepotentials of the signal lines 141, 142, and 143 are controlled by thesignal line driver circuit 12.

The pixel portion 10 includes a plurality of pixels 15 arranged inmatrix (3n rows and m columns). Each of the scan lines 131, 132, and 133is electrically connected to m pixels 15 arranged in a given row amongthe plurality of pixels 15 arranged in matrix (3n rows and m columns).Each of the signal lines 141, 142, and 143 is electrically connected to3n pixels 15 arranged in a given column among the plurality of pixels 15arranged in matrix (3n rows and m columns).

To the scan line driver circuit 11, start signals (GSP1 to GSP3) for thescan line driver circuit, a clock signal (GCK) for the scan line drivercircuit, and drive power supplies such as a high power supply potential(VDD) and a low power supply potential (VSS) are input from the outside.To the signal line driver circuit 12, signals such as a start signal(SSP) for the signal line driver circuit, a clock signal (SCK) for thesignal line driver circuit, and image signals (DATA1 to DATA3) and drivepower supplies such as a high power supply potential and a low powersupply potential are input from the outside.

FIG. 1B illustrates an example of a circuit configuration of the pixel15. The pixel 15 in FIG. 1B includes a transistor 151, a transistor 152,a transistor 153, a capacitor 154, and a liquid crystal element 155. Agate of the transistor 151 is electrically connected to the scan line131. One of a source and a drain of the transistor 151 is electricallyconnected to the signal line 141. A gate of the transistor 152 iselectrically connected to the scan line 132. One of a source and a drainof the transistor 152 is electrically connected to the signal line 142.A gate of the transistor 153 is electrically connected to the scan line133. One of a source and a drain of the transistor 153 is electricallyconnected to the signal line 143. One electrode of the capacitor 154 iselectrically connected to the other of the source and the drain of eachof the transistors 151 to 153. The other electrode of the capacitor 154is electrically connected to a wiring that supplies a capacitorpotential. One electrode of the liquid crystal element 155 iselectrically connected to the other of the source and the drain of eachof the transistors 151 to 153 and one electrode of the capacitor 154.The other electrode of the liquid crystal element 155 is electricallyconnected to a wiring that supplies a counter potential.

(Structural Example of Scan Line Driver Circuit 11)

FIG. 2 illustrates a structural example of the scan line driver circuit11 included in the liquid crystal display device in FIG. 1A. The scanline driver circuit 11 illustrated in FIG. 2 includes three shiftregisters 111 to 113 each having 3n output terminals. Each outputterminal of the shift register 111 is electrically connected to one ofthe 3n scan lines 131 placed in the pixel portion 10. Each outputterminal of the shift register 112 is electrically connected to one ofthe 3n scan lines 132 placed in the pixel portion 10. Each outputterminal of the shift register 113 is electrically connected to one ofthe 3n scan lines 133 placed in the pixel portion 10. In other words,the shift register 111 drives the scan lines 131; the shift register 112drives the scan lines 132; and the shift register 113 drives the scanlines 133. Specifically, the shift register 111 has a function ofsequentially shifting selection signals from the scan line 131 in afirst row (i.e., a function of sequentially selecting the scan lines 131every ½ cycle of the clock signal (GCK)) by using the first start signal(GSP1) input from the outside, as a trigger. The shift register 112 hasa function of sequentially shifting selection signals from the scan line132 in the first row, by using the second start signal (GSP2) input fromthe outside, as a trigger. The shift register 113 has a function ofsequentially shifting selection signals from the scan line 133 in thefirst row, by using the third start signal (GSP3) input from theoutside, as a trigger.

(Operation Example of Scan Line Driver Circuit 11)

An operation example of the scan line driver circuit 11 will bedescribed with reference to FIG. 3. FIG. 3 shows the clock signal (GCK),signals (SR111out) output from the 3n output terminals of the shiftregister 111, signals (SR112out) output from the 3n output terminals ofthe shift register 112, and signals (SR113out) output from the 3n outputterminals of the shift register 113. Here, a sampling period means aperiod required to input any image signals to all pixels arranged in allthe rows (from the first row to the 3n-th row).

In a sampling period (t1), in the shift register 111, high-levelpotentials are sequentially shifted every ½ clock cycle (everyhorizontal scan period) from the scan line 131 in the first row to thescan line 131 in an n-th row. In the shift register 112, high-levelpotentials are sequentially shifted every ½ clock cycle (everyhorizontal scan period) from the scan line 132 in an (n+1)th row to thescan line 132 in a 2n-th row. In the shift register 113, high-levelpotentials are sequentially shifted every ½ clock cycle (everyhorizontal scan period) from the scan line 133 in a (2n+1)th row to thescan line 133 in a 3n-th row. Accordingly, the scan line driver circuit11 sequentially selects m pixels 15 in the first row to m pixels 15 inthe n-th row through the scan lines 131, sequentially selects m pixels15 in the (n+1)th row to m pixels 15 in the 2n-th row through the scanlines 132, and sequentially selects m pixels 15 in the (2n+1)th row to mpixels 15 in the 3n-th row through the scan lines 133. That is, the scanline driver circuit 11 can supply selection signals to 3 m pixels 15provided in three different rows every horizontal scan period.

In a sampling period (t2), although output signals of the shiftregisters 111 to 113 are different from those in the sampling period(t1), the following operations are the same as those in the samplingperiod (W. That is, one of the shift registers 111 to 113 (the shiftregister 113 in the sampling period (t2)) sequentially selects m pixels15 in the first row to m pixels 15 in the n-th row; another one of theshift registers 111 to 113 (the shift register 111 in the samplingperiod (t2)) sequentially selects m pixels 15 in the (n+1)th row to mpixels 15 in the 2n-th row; and the other of the shift registers 111 to113 (the shift register 112 in the sampling period (t2)) sequentiallyselects m pixels 15 in the (2n+1)th row to m pixels 15 in the 3n-th row.In other words, as in the sampling period (t1), the scan line drivercircuit 11 can supply selection signals to 3 m pixels 15 in given threerows every horizontal scan period.

(Structural Example of Signal Line Driver Circuit 12)

FIG. 4A illustrates a structural example of the signal line drivercircuit 12 included in the liquid crystal display device in FIG. 1A. Thesignal line driver circuit 12 in FIG. 4A includes a shift register 120having m output terminals, m transistors 121, m transistors 122, and mtransistors 123. A gate of the transistor 121 is electrically connectedto the j-th output terminal (j is a natural number of 1 or more and m orless) of the shift register 120. One of a source and a drain of thetransistor 121 is electrically connected to a wiring that supplies thefirst image signal (DATA1). The other of the source and the drain of thetransistor 121 is electrically connected to the signal line 141 in thej-th column in the pixel portion 10. A gate of the transistor 122 iselectrically connected to the j-th output terminal of the shift register120. One of a source and a drain of the transistor 122 is electricallyconnected to a wiring that supplies the second image signal (DATA2). Theother of the source and the drain of the transistor 122 is electricallyconnected to the signal line 142 in the j-th column in the pixel portion10. A gate of the transistor 123 is electrically connected to the j-thoutput terminal of the shift register 120. One of a source and a drainof the transistor 123 is electrically connected to a wiring thatsupplies the third image signal (DATA3). The other of the source and thedrain of the transistor 123 is electrically connected to the signal line143 in the j-th column in the pixel portion 10.

Note that here, as the first image signal (DATA1), a red (R) imagesignal (an image signal for controlling transmission of red (R) light)is supplied to the signal line 141. As the second image signal (DATA2),a blue (B) image signal (an image signal for controlling transmission ofblue (B) light) is supplied to the signal line 142. As the third imagesignal (DATA3), a green (G) image signal (an image signal forcontrolling transmission of green (G) light) is supplied to the signalline 143.

(Structural Example of Backlight)

FIG. 4B illustrates a structural example of a backlight provided behindthe pixel portion 10 in the liquid crystal display device illustrated inFIG. 1A. The backlight in FIG. 4B includes a plurality of backlightunits 16 each including light sources that emit lights of three colorsof red (R), green (G), and blue (B). The plurality of backlight units 16are arranged in matrix, and lighting of the backlight units can becontrolled in each given region. Here, as the backlight for theplurality of pixels 15 of 3n rows and m columns, a backlight unit groupis provided at least every k rows and m columns (here, k is n/4), and itis possible to control lighting of these backlight unit groupsindependently. That is, the backlight includes at least a backlight unitgroup for first to k-th rows to a backlight unit group for (3n−k+1)th to3n-th rows, and it is possible to control lighting of each backlightunit group independently.

(Operation Example of Liquid Crystal Display Device)

FIG. 5 illustrates timings of lighting of the backlight unit group forthe first to k-th rows to the backlight unit group for the (3n−k+1)th to3n-th rows that are included in the backlight, and timings of supply ofimage signals to from m pixels in the first row to m pixels in the 3n-throw in the pixel portion 10 in the above liquid crystal display device.Specifically, in FIG. 5, “1” to “3n” indicate the number of rows andsolid lines indicate timing of input of image signals to rows. In theliquid crystal display device, image signals can be input to each pixelin the sampling period (t1) in the following manner: m pixels 15 in thefirst row to m pixels 15 in the n-th row are sequentially selected; mpixels 15 in the (n+1)th row to m pixels 15 in the 2n-th row aresequentially selected; and m pixels 15 in the (2n+1)th row to m pixels15 in the 3n-th row are sequentially selected. Specifically, in theliquid crystal display device, in the sampling period (t1), transistors151 included in the m pixels 15 in the first row to transistors 151included in the m pixels 15 in the n-th row are sequentially turned onthrough the scan lines 131 so that red (R) image signals can besequentially input to the pixels through the signal line 141;transistors 152 included in the m pixels 15 in the (n+1)th row totransistors 152 included in the m pixels 15 in the 2n-th row aresequentially turned on through the scan lines 132 so that blue (B) imagesignals can be sequentially input to the pixels through the signal line142; and transistors 153 included in the m pixels 15 in the (2n+1)th rowto transistors 153 included in the m pixels 15 in the 3n-th row aresequentially turned on through the scan lines 133 so that green (G)image signals can be sequentially input to the pixels through the signalline 143.

Further, in the liquid crystal display device, in the sampling period(t1), red (R) light can be emitted from the backlight unit group for thefirst to k-th rows after input of red (R) image signals to the m pixels15 in the first row to the m pixels 15 in the n-th row is finished; blue(B) light can be emitted from the backlight unit group for the (n+1)thto (n+k)th rows after input of blue (B) image signals to the m pixels 15in the (n+1)th row to the m pixels 15 in the (n+k)th row is finished;and green (G) light can be emitted from the backlight unit group for the(2n+1)th to (2n+k)th rows after input of green (G) image signals to them pixels 15 in the (2n+1)th row to the m pixels 15 in the 2(n+k)th rowis finished. That is, in the liquid crystal display device, supply ofselection signals and supply of light of a given color can beconcurrently performed in each region (in the first to n-th rows, the(n+1)th to 2n-th rows, and the (2n+1)th to 3n-th rows).

(Liquid Crystal Display Device disclosed in This Specification)

In the liquid crystal display device disclosed in this specification,image signals can be concurrently supplied to pixels placed in aplurality of rows among the pixels arranged in matrix. Thus, thefrequency of input of an image signal to each pixel can be increasedwithout change in response speed of a transistor or the like included inthe liquid crystal display device. Specifically, in the liquid crystaldisplay device, the frequency of input of an image signal to each pixelcan be tripled without change in clock frequency or the like of the scanline driver circuit. In other words, the liquid crystal display deviceis preferably applied to a field-sequential liquid crystal displaydevice or a liquid crystal display device driven by high frame ratedriving.

The liquid crystal display device disclosed in this specification ispreferably applied to a field-sequential liquid crystal display devicebecause of the following reasons. As described above, in afield-sequential liquid crystal display device, a display period isdivided between lights of given colors. For that reason, displayperceived by a user is sometimes changed (degraded) from display basedon original display data (such a phenomenon is also referred to as colorbreaks) because of a lack of a given piece of display data due totemporary interruption of display, such as a blink of the user. Anincrease in frame frequency is effective in reducing color breaks.Further, in order to display images by a field sequential method, thefrequency of input of an image signal to each pixel needs to be higherthan the frame frequency. For that reason, in the case where images aredisplayed with a field sequential method and high frame frequencydriving in a conventional liquid crystal display device, requirementsfor performance (high-speed response) of elements in the liquid crystaldisplay device are extremely strict. In contrast, in the liquid crystaldisplay device disclosed in this specification, the frequency of inputof an image signal to each pixel can be increased regardless ofcharacteristics of elements. Therefore, color breaks in thefield-sequential liquid crystal display device can be easily suppressed.

In addition, in the case where images are displayed by a fieldsequential method, it is preferable that backlight unit groupsconcurrently emit lights of different colors in regions as illustratedin FIG. 5 because of the following reasons. In the case where light ofone color is supplied for the entire screen, the pixel portion only hasdata on a specific color at a given moment. Therefore, a lack of displaydata in a given period due to a blink of the user or the likecorresponds to a lack of data on a specific color. In contrast, in thecase where backlight units emit lights of different colors in regions,the pixel portion has data on the colors at a given moment. Therefore, alack of display data in a given period due to a blink of the user or thelike does not correspond to a lack of data on a specific color. In otherwords, color breaks can be reduced when backlight units emit lights ofdifferent colors in regions. Furthermore, in the case where thebacklight unit groups are lit as illustrated in FIG. 5, adjacentbacklight unit groups do not emit light of a different color.Specifically, in the sampling period (t1), when the backlight unit groupfor the (n+1)th to (n+k)th rows emits blue (B) light after input of blue(B) image signals to the m pixels 15 in the (n+1)th row to the m pixels15 in the (n+k)th row is finished, the backlight unit group for the(3k+1)th to n-th rows and the backlight unit group for the (n+k+1)th to(n+2k)th rows emit blue (B) light or do not emit light at all (i.e., donot emit either red (R) light or green (G) light). Thus, it is possibleto decrease the probability that light with a color other than aspecific color is transmitted through a pixel to which image data of thespecific color is input.

(Variations)

The liquid crystal display device having the above-described structureis one embodiment of the present invention; the present inventionfurther includes a liquid crystal display device that is different fromthe liquid crystal display device having the above-described structure.

For example, the above liquid crystal display device has the structurein which image signals are concurrently supplied to 3 m pixels in giventhree rows in the pixel portion 10; however, the liquid crystal displaydevice of the present invention is not limited to having this structure.That is, in the liquid crystal display device of the present invention,image signals can be concurrently supplied to a plurality of pixels ingiven plural rows in the pixel portion 10. Note that it is obvious thatin the case where the number of rows is changed, the number of shiftregisters and the number of rows need to the same.

The liquid crystal display device has the structure in which imagesignals are concurrently supplied to pixels in given three rows providedat regular intervals (the interval between the rows supplied with imagesignals corresponds to n rows of pixels); however, the liquid crystaldisplay device of the present invention is not limited to having thisstructure. That is, the liquid crystal display device of the presentinvention can have a structure in which image signals are concurrentlysupplied to pixels in given three rows provided at irregular intervals.Specifically, the liquid crystal display device can have a structure inwhich image signals are concurrently supplied to m pixels in the firstrow, m pixels in an (a+1)th row (a is a natural number), and m pixels inan (a+b+1)th row (b is a natural number other than a).

Moreover, in the liquid crystal display device, the scan line drivercircuit is constituted by a shift register; the shift register can bereplaced with a circuit having an equivalent function. For example, theshift register can be replaced with a decoder.

In the liquid crystal display device, light sources each emitting one ofred (R) light, green (G) light, and blue (B) light are used for thebacklight; however, the liquid crystal display device of the presentinvention is not limited to having this structure. That is, in theliquid crystal display device of the present invention, light sourcesthat emit lights of given colors can be used in combination. Forexample, it is possible to use a combination of four kinds of lightsources of red (R), green (G), blue (B), and white (W); or a combinationof three kinds of light sources of cyan, magenta, and yellow. Moreover,it is possible to use a combination of six kinds of light sources ofpale red (R), pale green (G), pale blue (B), dark red (R), dark green(G), and dark blue (B); or a combination of six kinds of light sourcesof red (R), green (G), blue (B), cyan, magenta, and yellow. In such amanner, with a combination of lights of a wider variety of colors, thecolor gamut of the liquid crystal display device can be enlarged, andthe image quality can be improved.

The liquid crystal display device includes the capacitor for holding avoltage applied to the liquid crystal element (see FIG. 1B);alternatively, it is possible to employ a structure in which thecapacitor is not provided.

In the liquid crystal display device, light sources that emit lights ofthree colors of red (R), green (G), and blue (B) are aligned linearlyand horizontally as the backlight unit (see FIG. 4B); however, thestructure of the backlight unit is not limited to this. For example,light sources that emit lights of three colors may be arranged intriangle or aligned linearly and vertically, or a red (R) light source,a green (G) light source, and a blue (B) light source may be separatelyprovided. Further, the liquid crystal display device includes adirect-type backlight as the backlight (see FIG. 4B); alternatively, anedge-lit backlight can be used as the backlight.

Embodiment 2

In this embodiment, an example of a field-sequential liquid crystaldisplay device having a structure different from that in Embodiment 1will be described with reference to FIGS. 6A to 6D, FIGS. 7A and 7B, andFIG. 8.

(Structural Example of Liquid Crystal Display Device)

FIG. 6A illustrates a structural example of a liquid crystal displaydevice. The liquid crystal display device in FIG. 6A includes a pixelportion 30; a scan line driver circuit 31; a signal line driver circuit32; 3n scan lines 33 (n is a natural number of 2 or more) arrangedparallel or approximately parallel to each other; and m signal lines341, m signal lines 342, and m signal lines 343 (m is a natural numberof 2 or more) arranged parallel or approximately parallel to each other.The potentials of the scan lines 33 are controlled by the scan linedriver circuit 31. The potentials of the signal lines 341, 342, and 343are controlled by the signal line driver circuit 32.

The pixel portion 30 is divided into three regions (regions 301 to 303),and each region includes a plurality of pixels arranged in matrix (nrows and m columns). Each of the scan lines 33 is electrically connectedto m pixels arranged in a given row among the plurality of pixelsarranged in matrix (3n rows and m columns) in the pixel portion 30. Eachof the signal lines 341 is electrically connected to n pixels arrangedin a given column among the plurality of pixels arranged in matrix (nrows and m columns) in the region 301. Each of the signal lines 342 iselectrically connected to n pixels arranged in a given column among theplurality of pixels arranged in matrix (n rows and m columns) in theregion 302. Each of the signal lines 343 is electrically connected to npixels arranged in a given column among the plurality of pixels arrangedin matrix (n rows and m columns) in the region 303.

To the scan line driver circuit 31, a start signal (GSP) for the scanline driver circuit, a clock signal (GCK) for the scan line drivercircuit, and drive power supplies such as a high power supply potentialand a low power supply potential are input from the outside. To thesignal line driver circuit 32, signals such as a start signal (SSP) forthe signal line driver circuit, a clock signal (SCK) for the signal linedriver circuit, and image signals (data1 to data3) and drive powersupplies such as a high power supply potential and a low power supplypotential are input from the outside.

FIGS. 6B to 6D each illustrate an example of a circuit configuration ofthe pixel. Specifically, FIG. 6B illustrates an example of a circuitconfiguration of a pixel 351 placed in the region 301; FIG. 6Cillustrates an example of a circuit configuration of a pixel 352 placedin the region 302; and FIG. 6D illustrates an example of a circuitconfiguration of a pixel 353 placed in the region 303. The pixel 351 inFIG. 6B includes a transistor 3511, a capacitor 3512, and a liquidcrystal element 3514. A gate of the transistor 3511 is electricallyconnected to the scan line 33. One of a source and a drain of thetransistor 3511 is electrically connected to the signal line 341. Oneelectrode of the capacitor 3512 is electrically connected to the otherof the source and the drain of the transistor 3511. The other electrodeof the capacitor 3512 is electrically connected to a wiring thatsupplies a capacitor potential. One electrode of the liquid crystalelement 3514 is electrically connected to the other of the source andthe drain of the transistor 3511 and one electrode of the capacitor3512. The other electrode of the liquid crystal element 3514 iselectrically connected to a wiring that supplies a counter potential.

The pixel 352 in FIG. 6C and the pixel 353 in FIG. 6D have the samecircuit configuration as the pixel 351 in FIG. 6B. Note that the pixel352 in FIG. 6C is different from the pixel 351 in FIG. 6B in that one ofa source and a drain of a transistor 3521 is electrically connected tothe signal line 342 instead of the signal line 341. The pixel 353 inFIG. 6D is different from the pixel 351 in FIG. 6B in that one of asource and a drain of the transistor 3531 is electrically connected tothe signal line 343 instead of the signal line 341.

(Structural Example of Scan Line Driver Circuit 31)

FIG. 7A illustrates a structural example of the scan line driver circuit31 included in the liquid crystal display device in FIG. 6A. The scanline driver circuit 31 illustrated in FIG. 7A includes three shiftregisters 311 to 313 each having n output terminals. Each outputterminal of the shift register 311 is electrically connected to one of nscan lines 33 placed in the region 301. Each output terminal of theshift register 312 is electrically connected to one of n scan lines 33placed in the region 302. Each output terminal of the shift register 313is electrically connected to one of n scan lines 33 placed in the region303. In other words, the shift register 311 supplies selection signalsin the region 301; the shift register 312 supplies selection signals inthe region 302; and the shift register 313 supplies selection signals inthe region 303. Specifically, the shift register 311 has a function ofsequentially shifting selection signals from the scan line 33 in a firstrow (i.e., a function of sequentially selecting the scan lines 33 every½ cycle of the clock signal (GCK)) by using the start signal (GSP) inputfrom the outside, as a trigger. The shift register 312 has a function ofsequentially shifting selection signals from the scan line 33 in an(n+1)-th row, by using the start signal (GSP) input from the outside, asa trigger. The shift register 313 has a function of sequentiallyshifting selection signals from the scan line 33 in a (2n+1)-th row, byusing the start signal (GSP) input from the outside, as a trigger.

(Operation Example of Scan Line Driver Circuit 31)

An operation example of the scan line driver circuit 31 will bedescribed with reference to FIG. 7B. FIG. 7B shows the clock signal(GCK), signals (SR311out) output from the n output terminals of theshift register 311, signals (SR312out) output from the n outputterminals of the shift register 312, and signals (SR313out) output fromthe n output terminals of the shift register 313.

In a sampling period (T1), in the shift register 311, high-levelpotentials are sequentially shifted every ½ clock cycle (everyhorizontal scan period) from the scan line 33 in the first row to thescan line 33 in an n-th row. In the shift register 312, high-levelpotentials are sequentially shifted every ½ clock cycle (everyhorizontal scan period) from the scan line 33 in an (n+1)th row to thescan line 33 in a 2n-th row. In the shift register 313, high-levelpotentials are sequentially shifted every ½ clock cycle (everyhorizontal scan period) from the scan line 33 in a (2n+1)th row to thescan line 33 in a 3n-th row. Accordingly, the scan line driver circuit31 sequentially selects m pixels 351 in the first row to m pixels 351 inthe n-th row, sequentially selects m pixels 352 in the (n+1)th row to mpixels 352 in the 2n-th row, and sequentially selects m pixels 353 inthe (2n+1)th row to m pixels 353 in the 3n-th row through the scan lines33. That is, the scan line driver circuit 31 can supply selectionsignals to 3 m pixels provided in three different rows every horizontalscan period.

In a sampling period (T2) and a sampling period (T3), the operations ofthe shift registers 311 to 313 are the same as the operations in thesampling period (T1). That is, as in the sampling period (T1), the scanline driver circuit 31 can supply selection signals to 3 m pixelsprovided in given three rows every horizontal scan period.

(Structural Example of Signal Line Driver Circuit 32)

FIG. 8 illustrates a structural example of the signal line drivercircuit 32 included in the liquid crystal display device in FIG. 6A. Thesignal line driver circuit 32 in FIG. 8 includes a shift register 320having m output terminals, m transistors 321, m transistors 322, and mtransistors 323. A gate of the transistor 321 is electrically connectedto the j-th output terminal (j is a natural number of 1 to m) of theshift register 320. One of a source and a drain of the transistor 321 iselectrically connected to a wiring that supplies the first image signal(data1). The other of the source and the drain of the transistor 321 iselectrically connected to the signal line 341 in the j-th column in thepixel portion 30. A gate of the transistor 322 is electrically connectedto the j-th output terminal of the shift register 320. One of a sourceand a drain of the transistor 322 is electrically connected to a wiringthat supplies the second image signal (data2). The other of the sourceand the drain of the transistor 322 is electrically connected to thesignal line 342 in the j-th column in the pixel portion 30. A gate ofthe transistor 323 is electrically connected to the j-th output terminalof the shift register 320. One of a source and a drain of the transistor323 is electrically connected to a wiring that supplies the third imagesignal (data3). The other of the source and the drain of the transistor323 is electrically connected to the signal line 343 in the j-th columnin the pixel portion 30.

Note that here, as the first image signal (data1), a red (R) imagesignal (an image signal for controlling transmission of red (R) light),a green (G) image signal (an image signal for controlling transmissionof green (G) light), and a blue (B) image signal (an image signal forcontrolling transmission of blue (B) light) are supplied to the signalline 341 in the sampling period (T1), the sampling period (T2), and thesampling period (T3), respectively. As the second image signal (data2),a blue (B) image signal, a red (R) image signal, and a green (G) imagesignal are supplied to the signal line 342 in the sampling period (T1),the sampling period (T2), and the sampling period (T3), respectively. Asthe third image signal (data3), a green (G) image signal, a blue (B)image signal, and a red (R) image signal are supplied to the signal line343 in the sampling period (T1), the sampling period (T2), and thesampling period (T3), respectively.

(Structural Example of Backlight)

As a backlight of the liquid crystal display device in this embodiment,the backlight shown in Embodiment 1 can be used; therefore, the abovedescription is to be referred to here.

(Operation Example of Liquid Crystal Display Device)

The liquid crystal display device in this embodiment can performoperations similar to those of the liquid crystal display device inEmbodiment 1 (see FIG. 5). That is, in the liquid crystal display devicein this embodiment, image signals can be input to each pixel in thesampling period (T1) in the following manner: m pixels 351 in the firstrow to m pixels 351 in the n-th row are sequentially selected, m pixels352 in the (n+1)th row to m pixels 352 in the 2n-th row are sequentiallyselected, and m pixels 353 in the (2n+1)th row to m pixels 353 in the3n-th row are sequentially selected.

As in the liquid crystal display device shown in Embodiment 1, in theliquid crystal display device of this embodiment, supply of selectionsignals and supply of light of a given color can be concurrentlyperformed in each region (in the first to n-th rows, the (n+1)th to2n-th rows, and the (2n+1)th to 3n-th rows).

(Liquid Crystal Display Device in This Embodiment)

The liquid crystal display device in this embodiment has functionssimilar to those of the liquid crystal display device described inEmbodiment 1. Further, the aperture ratio of the liquid crystal displaydevice in this embodiment can be increased by the reduction in thenumber of scan lines provided in the pixel portion and the reduction inthe number of transistors provided in each pixel, as compared to theliquid crystal display device in Embodiment 1. Moreover, the reductionin the number of scan lines provided in the pixel portion can reduceparasitic capacitance generated when a signal line and a scan lineoverlap with each other; thus, the signal line can be driven at highspeed. In addition, the area of the scan line driver circuit can bereduced, and the number of signals necessary for operating the scan linedriver circuit can be reduced (i.e., it is not necessary to supplydifferent start signals for the scan line driver circuit to a pluralityof shift registers).

(Variations)

The liquid crystal display device in this embodiment is one embodimentof the present invention; the present invention further includes aliquid crystal display device that is different from the liquid crystaldisplay device. For example, the liquid crystal display device in thisembodiment can have any of the structures shown as the variations inEmbodiment 1. Specifically, the shift register included in the liquidcrystal display device in this embodiment can be replaced with a circuithaving an equivalent function (e.g., a decoder), for example.

In addition, the liquid crystal display device in this embodiment hasthe structure in which the pixel portion 30 is divided into threeregions; however, the liquid crystal display device in this embodimentis not limited to having this structure. That is, in the liquid crystaldisplay device in this embodiment, the pixel portion 30 can be dividedinto given plural regions. Note that it is obvious that in the casewhere the number of regions is changed, the number of shift registersand the number of rows need to the same.

In the liquid crystal display device in this embodiment, the number ofpixels is the same in three regions (i.e., each of the regions includespixels of n rows and m columns); alternatively, the number of pixels canvary between regions. Specifically, a first region can include pixels ofc rows and m columns (c is a natural number) and a second region caninclude pixels of d rows and m columns (d is a natural number other thanc).

Embodiment 3

In this embodiment, a specific structure of the liquid crystal displaydevice described in Embodiment 1 or 2 will be described.

(Example of Transistor provided in Pixel)

In the liquid crystal display device in Embodiment 1, a plurality oftransistors are provided in each pixel. Input of an image signal to thepixel is controlled by sequentially using the plurality of transistors.Thus, image signals can be concurrently supplied to pixels placed in aplurality of rows. Note that in the liquid crystal display device, theleakage of an image signal held at a pixel is accordingly increased inaccordance with the increase in the number of transistors provided inthe pixel. For that reason, in the liquid crystal display device, atransistor with excellent off-state characteristics (with low off-statecurrent) is preferably used as the transistor provided in each pixel. Anexample of a transistor that is suitable for the transistor will bedescribed below with reference to FIG. 9. Specifically, a transistorincluding a channel formation region formed using an oxide semiconductorwill be described. The off-state current of the transistor can beextremely reduced when the oxide semiconductor is purified (which isdescribed below in detail). The transistor can also be used for formingthe scan line driver circuit. In that case, reduction in cost andincrease in yield due to reduction in the number of manufacturing stepscan be achieved.

Note that the band gap of the oxide semiconductor is 3.0 eV to 3.5 eV.The band gap of silicon carbide and the band gap of gallium nitride are3.26 eV and 3.39 eV, respectively, which are about three times that ofsilicon. Therefore, compound semiconductors such as silicon carbide andgallium nitride are similar to the oxide semiconductor in that they areboth wide band gap semiconductors. The wider band gap is advantageous inincreasing the withstand voltage of a signal processing circuit,reducing loss of power, and the like.

Compound semiconductors such as silicon carbide and gallium nitride arerequired to be single crystal, and it is difficult to meet themanufacturing conditions for obtaining a single crystal material; forexample, crystal needs to grow at a temperature that is much higher thanthe process temperature of the oxide semiconductor, and epitaxial growthover a special substrate is necessary. Such conditions do not allow filmformation of any of these compound semiconductors over a silicon waferthat can be obtained easily or a glass substrate whose allowabletemperature limit is low. Therefore, an inexpensive substrate cannot beused, and further, the substrate cannot be increased in size, so thatthe productivity of signal processing circuits using the compoundsemiconductor such as silicon carbide or gallium nitride is low. Incontrast, the oxide semiconductor can be deposited with heat treatmentat 300° C. to 850° C., that is, can be deposited over a glass substrate.Moreover, a semiconductor element formed using the oxide semiconductorcan be stacked over an integrated circuit.

A transistor 211 illustrated in FIG. 9 includes a gate layer 221provided over a substrate 220 having an insulating surface, a gateinsulating layer 222 provided over the gate layer 221, an oxidesemiconductor layer 223 provided over the gate insulating layer 222, anda source layer 224 a and a drain layer 224 b provided over the oxidesemiconductor layer 223. Moreover, FIG. 9 illustrates an insulatinglayer 225 that covers the transistor 211 and is in contact with theoxide semiconductor layer 223, and a protective insulating layer 226provided over the insulating layer 225.

As described above, the transistor 211 in FIG. 9 includes the oxidesemiconductor layer 223 as a semiconductor layer. Examples of an oxidesemiconductor used for the oxide semiconductor layer 223 are anIn—Sn—Ga—Zn—O-based oxide semiconductor which is an oxide of four metalelements; an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-basedoxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, and a Sn—Al—Zn—O-based oxide semiconductor which areoxides of three metal elements; an In—Ga—O-based oxide semiconductor, anIn—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor,an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxidesemiconductor, a Sn—Mg—O-based oxide semiconductor, and an In—Mg—O-basedoxide semiconductor which are oxides of two metal elements; and anIn—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, and aZn—O-based oxide semiconductor which are oxides of one metal element.Further, SiO₂ may be contained in the above oxide semiconductor. Here,for example, an In—Ga—Zn—O-based oxide semiconductor is an oxidecontaining at least In, Ga, and Zn, and there is no particularlimitation on the composition ratio of the elements. An In—Ga—Zn—O-basedoxide semiconductor may contain an element other than In, Ga, and Zn.

As the oxide semiconductor layer 223, a thin film expressed by achemical formula of InMO₃(ZnO)_(m) (m>0) can be used. Here, M representsone or more metal elements selected from Ga, Al, Mn, and Co. Forexample, M can be Ga, Ga and Al, Ga and Mn, or Ga and Co.

In the case where an In—Zn—O-based material is used as an oxidesemiconductor, a target to be used has a composition ratio of In:Zn=50:1to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably In:Zn=15:1 to 1.5:1 in an atomicratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, when atarget used for forming an In—Zn—O-based oxide semiconductor has anatomic ratio of In:Zn:O=X:Y:Z, the relation of Z>(1.5X+Y) is satisfied.

The above-described oxide semiconductor is an oxide semiconductor thatis purified and made to be electrically i-type (intrinsic) as follows:an impurity such as hydrogen, moisture, a hydroxyl group, or hydride(also referred to as a hydrogen compound), which is a factor ofvariation in electric characteristics, is intentionally eliminated inorder to prevent variation in electric characteristics.

Therefore, it is preferable that the oxide semiconductor contain aslittle hydrogen as possible. Moreover, the number of carriers derivedfrom hydrogen, oxygen vacancy, and the like is extremely small (close tozero) in the purified oxide semiconductor layer, and the carrier densityis less than 1×10¹²/cm³, preferably less than 1×10¹¹/cm³. In otherwords, the density of carriers derived from hydrogen, oxygen vacancy,and the like in the oxide semiconductor layer is made as close to zeroas possible. Since the oxide semiconductor layer has very few carriersderived from hydrogen, oxygen vacancy, and the like, the amount ofleakage current at the time when the transistor is off (i.e., off-statecurrent) can be small. Furthermore, since the number of impurity levelsderived from hydrogen, oxygen vacancy, and the like is small, variationand deterioration of electric characteristics due to light irradiation,temperature change, application of bias, or the like can be reduced.Note that the smaller the amount of off-state current is, the better.The transistor including the oxide semiconductor for a semiconductorlayer has an off-state current value (per channel width (W) of 1 μm) of100 zA (zeptoamperes) or less, preferably 10 zA or less, more preferably1 zA or less. Further, the transistor does not have PN junction and hotcarrier degradation does not occur, so that electrical characteristicsof the transistor are not adversely affected thereby.

The off-state current can be extremely low in a transistor in which anoxide semiconductor layer that is purified by drastically removinghydrogen contained therein as described above is used for a channelformation region. In other words, in circuit design, the oxidesemiconductor layer can be considered as an insulator when thetransistor is off. On the other hand, when the transistor is on, thecurrent supply capability of the oxide semiconductor layer is expectedto be higher than that of a semiconductor layer formed of amorphoussilicon.

As the substrate 220 having an insulating surface, a glass substrate ofbarium borosilicate glass, aluminoborosilicate glass, or the like can beused, for example.

In the transistor 211, an insulating film serving as a base film may beprovided between the substrate 220 and the gate layer 221. The base filmhas a function of preventing diffusion of an impurity element from thesubstrate, and can be formed with a single-layer structure or a stackedstructure using one or more of a silicon nitride film, a silicon oxidefilm, a silicon nitride oxide film, and a silicon oxynitride film.

The gate layer 221 can be formed with a single-layer structure or astacked structure using a metal material such as molybdenum, titanium,chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandiumor an alloy material containing any of these materials as its maincomponent.

The gate insulating layer 222 can be formed with a single-layerstructure or a stacked structure including a silicon oxide layer, asilicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, an aluminum oxide layer, an aluminum nitride layer, analuminum oxynitride layer, an aluminum nitride oxide layer, or a hafniumoxide layer by plasma CVD, sputtering, or the like. For example, asilicon nitride layer (SiNy (y>0)) with a thickness of 50 nm to 200 nmcan be formed as a first gate insulating layer by plasma CVD, and asilicon oxide layer (SiOx (x>0)) with a thickness of 5 nm to 300 nm as asecond gate insulating layer can be stacked over the first gateinsulating layer.

A conductive film used for the source layer 224 a and the drain layer224 b can be formed using an element selected from Al, Cr, Cu, Ta, Ti,Mo, and W, an alloy including any of these elements as a component, oran alloy film including a combination of any of these elements, forexample. A structure may be employed in which a refractory metal layerof Ti, Mo, W, or the like is stacked on one or both of a top surface anda bottom surface of a metal layer of Al, Cu, or the like. By using analuminum material to which an element preventing generation of hillocksand whiskers in an aluminum film (e.g., Si, Nd, or Sc) is added, heatresistance can be increased.

The conductive film to be the source layer 224 a and the drain layer 224b (including a wiring layer formed using the same layer as the sourceand drain layers) may be formed using a conductive metal oxide. As theconductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an alloy of indium oxide and tin oxide (In₂O₃—SnO₂,referred to as ITO), an alloy of indium oxide and zinc oxide(In₂O₃—ZnO), or any of the metal oxide materials containing silicon orsilicon oxide can be used.

As the insulating layer 225, an inorganic insulating film typified by asilicon oxide film, a silicon oxynitride film, an aluminum oxide film,or an aluminum oxynitride film can be used.

As the protective insulating layer 226, an inorganic insulating filmsuch as a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, or an aluminum nitride oxide film can be used.

A planarization insulating film may be formed over the protectiveinsulating layer 226 in order to reduce surface roughness due to thetransistor. The planarization insulating film can be formed using anorganic material such as polyimide, acrylic, or benzocyclobutene. Otherthan such organic materials, it is possible to use a low-dielectricconstant material (low-k material) or the like. Note that theplanarization insulating film may be formed by stacking a plurality ofinsulating films formed from these materials.

(Off-State Current of Transistor)

Next, results of measuring the off-state current of a transistorincluding a purified oxide semiconductor layer will be described.

First, a transistor with a sufficiently large channel width W of 1 m wasprepared in consideration of the fact that the transistor including apurified oxide semiconductor layer has an adequately low off-statecurrent, and the off-state current was measured. FIG. 10 shows theresults of measuring the off-state current of a transistor with achannel width W of 1 m. In FIG. 10, the horizontal axis represents agate voltage VG, and the vertical axis represents a drain current ID. Inthe case where the drain voltage VD is +1 V or +10 V and the gatevoltage VG is within the range of −20 V to −5 V, the off-state currentof the transistor was found to be less than or equal to 1×10⁻¹² A whichis the detection limit. Moreover, it was found that the off-statecurrent (here, per channel width of 1 μm) of the transistor was 1 aA/μm(1×10⁻¹⁸ A/μm) or less.

Next, the results of more accurately measuring the off-state current ofthe transistor including a purified oxide semiconductor layer will bedescribed. As described above, the off-state current of the transistorincluding a purified oxide semiconductor layer was found to be less thanor equal to 1×10⁻¹² A, which is the detection limit of measurementequipment. Here, the results of measuring more accurate off-statecurrent (the value smaller than or equal to the detection limit ofmeasurement equipment in the above measurement) with the use of anelement for evaluating characteristics will be described.

First, an element for evaluating characteristics which was used formeasuring current will be described with reference to FIG. 11.

In the element for evaluating characteristics illustrated in FIG. 11,three measurement systems 1800 are connected in parallel. Themeasurement system 1800 includes a capacitor 1802, a transistor 1804, atransistor 1805, a transistor 1806, and a transistor 1808. Thetransistor including a purified oxide semiconductor layer was used asthe transistors 1804 and 1808.

In the measurement system 1800, one of a source and a drain of thetransistor 1804, one terminal of the capacitor 1802, and one of a sourceand a drain of the transistor 1805 are connected to a power source (forsupplying V2). The other of the source and the drain of the transistor1804, one of a source and a drain of the transistor 1808, the otherterminal of the capacitor 1802, and a gate of the transistor 1805 areelectrically connected to each other. The other of the source and thedrain of the transistor 1808, one of a source and a drain of thetransistor 1806, and a gate of the transistor 1806 are electricallyconnected to a power source (for supplying V1). The other of the sourceand the drain of the transistor 1805 and the other of the source and thedrain of the transistor 1806 are electrically connected to an outputterminal.

A potential Vext_b2 for controlling the on/off state of the transistor1804 is supplied to a gate of the transistor 1804. A potential Vext_b1for controlling the on/off state of the transistor 1808 is supplied to agate of the transistor 1808. A potential Vout is output from the outputterminal.

Next, a method for measuring current with the use of the element forevaluating characteristics will be described with reference to FIG. 12.The measurement is performed in an initial period and a measurementperiod.

First, in the initial period, a node A (a node electrically connected toone of the source and the drain of the transistor 1808, the otherterminal of the capacitor 1802, and the gate of the transistor 1805) ismade to have a high potential. In order to realize this, the potentialof V1 is set to a high potential (VDD) and the potential of V2 is set toa low potential (VSS).

Next, Vext_b2 is set to a potential with which the transistor 1804 isturned on (a high potential). Thus, the potential of the node A becomesV2, that is, the low potential (VSS). Note that it is not alwaysnecessary to supply the low potential (VSS) to the node A. After that,Vext_b2 is set to a potential with which the transistor 1804 is turnedoff (a low potential), so that the transistor 1804 is turned off. Then,Vext_b1 is set to a potential with which the transistor 1808 is turnedon (a high potential). Thus, the potential of the node A becomes V1,that is, the high potential (VDD). After that, Vext_b1 is set to apotential with which the transistor 1808 is turned off. Accordingly, thenode A is brought into a floating state while having the high potential,and the initial period is completed.

In the following measurement period, the potential V1 and the potentialV2 are set to a potential with which electric charge flows to the node Aor a potential with which electric charge flows from the node A. Here,each of the potential V1 and the potential V2 is set to the lowpotential. Note that at the timing of measuring the output potentialVout, V1 is temporarily set to the high potential because an outputcircuit needs to be operated. The period in which V1 is the highpotential is made short so as not to adversely affect the measurement.

In the measurement period, electric charge is transferred from the nodeA to a wiring supplied with V1 or a wiring supplied with V2 because ofthe off-state current of the transistors 1804 and 1808. That is, theamount of electric charge held at the node A is changed over time, andthe potential of the node A is changed accordingly. This means that thepotential of the gate of the transistor 1805 varies.

Electric charge is measured by measuring Vout while the potential ofVext_b1 is temporarily set to the high potential at regular intervals. Acircuit constituted by the transistor 1805 and the transistor 1806 is aninverter. When the node A has the high potential, Vout becomes the lowpotential; whereas when the node A has the low potential, Vout becomesthe high potential. The potential of the node A, which has been the highpotential at the beginning, is gradually decreased because of a decreasein the amount of electric charge. Thus, the potential of Vout is alsochanged. The potential of the node A is amplified with the amplificationfunction of the inverter and output as Vout, so that a small amount ofchange in the potential of the node A can be measured by measurement ofVout.

A method for calculating the off-state current from the obtained outputpotential Vout will be described below.

The relation between the potential V_(A) of the node A and the outputpotential Vout is obtained in advance before calculation of theoff-state current. With this relation, the potential V_(A) of the node Acan be obtained using the output potential Vout. In accordance with theabove relation, the potential V_(A) of the node A can be expressed as afunction of the output potential Vout by the following equation.

V _(A) =F(Vout)  [Formula 1]

Electric charge Q_(A) of the node A can be expressed by the followingequation with the use of the potential V_(A) of the node A, acapacitance C_(A) connected to the node A, and a constant (const). Here,the capacitance C_(A) connected to the node A is the sum of thecapacitance of the capacitor 1802 and other capacitance.

Q _(A) =C _(A) V _(A)+const  [Formula 2]

Since a current I_(A) at the node A is obtained by time derivative ofelectric charge flowing to a capacitor connected to the node A (orelectric charge flowing from the capacitor connected to the node A), thecurrent I_(A) at the node A is expressed by the following equation.

$\begin{matrix}{{I_{A} \equiv \frac{\Delta \; Q_{A}}{\Delta \; t}} = \frac{{C_{A} \cdot \Delta}\; {F({Vout})}}{\Delta \; t}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In this manner, the current I_(A) of the node A can be obtained from thecapacitance C_(A) connected to the node A and the output potential Voutof the output terminal.

By the above method, it is possible to measure a leakage current whichflows between a source and a drain of a transistor in an off state (anoff-state current).

Here, the transistor 1804 and the transistor 1808 each of which includeda purified oxide semiconductor layer and had a channel length L of 10 μmand a channel width W of 50 μm were manufactured. In the measurementsystems 1800 arranged in parallel, capacitance values of the capacitors1802 were 100 fF, 1 pF, and 3 pF.

Note that in the above-described measurement, VDD was 5 V and VSS was 0V. In the measurement period, Vout was measured while the potential V1was basically VSS and set to VDD only in a period of 100 millisecondsevery 10 to 300 seconds. Moreover, At used in calculation of a current Iwhich flows through the element was about 30000 seconds.

FIG. 13 shows the relation between elapsed time Time in the currentmeasurement and the output potential Vout. It can be seen from FIG. 13that the potential is changed over time.

FIG. 14 shows the off-state current at room temperature (25° C.)calculated in the above current measurement. FIG. 14 shows the relationbetween a source-drain voltage V of the transistor 1804 or thetransistor 1808 and an off-state current I. It is found from FIG. 14that the off-state current is about 40 zA/μm when the source-drainvoltage is 4 V. In addition, the off-state current was 10 zA/μm or lesswhen the source-drain voltage was 3.1 V. Note that 1 zA represents 10⁻²¹A.

FIG. 15 shows the off-state current at 85° C. calculated in the abovecurrent measurement. FIG. 15 shows the relation between a source-drainvoltage V of the transistor 1804 or the transistor 1808 and an off-statecurrent I at 85° C. It is found from FIG. 15 that the off-state currentwas 100 zA/μm or less when the source-drain voltage was 3.1 V.

As has been described above, it was confirmed that the off-state currentwas sufficiently low in a transistor including a purified oxidesemiconductor layer.

(Variations of Transistor)

In the above description, the transistor 211 with a bottom-gatestructure called a channel-etch structure (see FIG. 9) is used as thetransistor provided in the pixel; however, the transistor is not limitedto having this structure. Transistors illustrated in FIGS. 16A to 16Ccan be used, for example.

A transistor 510 illustrated in FIG. 16A has a kind of bottom-gatestructure called a channel-protective type (also referred to as achannel-stop type).

The transistor 510 includes, over a substrate 220 having an insulatingsurface, a gate layer 221, a gate insulating layer 222, an oxidesemiconductor layer 223, an insulating layer 511 functioning as achannel protective layer that covers a channel formation region of theoxide semiconductor layer 223, a source layer 224 a, and a drain layer224 b. Moreover, a protective insulating layer 226 that covers thesource layer 224 a, the drain layer 224 b, and the insulating layer 511is formed.

As the insulating layer 511, an insulator such as silicon oxide, siliconnitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, ortantalum oxide can be used. A stacked structure of any of thesematerials can also be used.

A transistor 520 illustrated in FIG. 16B is a bottom-gate transistor.The transistor 520 includes, over a substrate 220 having an insulatingsurface, a gate layer 221, a gate insulating layer 222, a source layer224 a, a drain layer 224 b, and an oxide semiconductor layer 223.Furthermore, an insulating layer 225 that covers the source layer 224 aand the drain layer 224 b and is in contact with the oxide semiconductorlayer 223 is provided. A protective insulating layer 226 is providedover the insulating layer 225.

In the transistor 520, the gate insulating layer 222 is provided on andin contact with the substrate 220 and the gate layer 221, and the sourcelayer 224 a and the drain layer 224 b are provided on and in contactwith the gate insulating layer 222. Further, the oxide semiconductorlayer 223 is provided over the gate insulating layer 222, the sourcelayer 224 a, and the drain layer 224 b.

A transistor 530 illustrated in FIG. 16C is a kind of top-gatetransistor. The transistor 530 includes, over a substrate 220 having aninsulating surface, an insulating layer 531, an oxide semiconductorlayer 223, a source layer 224 a and a drain layer 224 b, a gateinsulating layer 222, and a gate layer 221. A wiring layer 532 a and awiring layer 532 b are provided in contact with the source layer 224 aand the drain layer 224 b, to be electrically connected to the sourcelayer 224 a and the drain layer 224 b, respectively.

As the insulating layer 531, an insulator such as silicon oxide, siliconnitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, ortantalum oxide can be used. A stacked structure of any of thesematerials can also be used.

As the wiring layers 532 a and 532 b, an element selected from aluminum(Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc); analloy containing any of these elements; or a nitride containing any ofthese elements can be used. A stacked structure of any of thesematerials can also be used.

(Specific Example of Cross Section of Pixel)

A liquid crystal display device according to one embodiment of thepresent invention can have high visibility and high reliability becausea highly reliable transistor with low off-state current is used in apixel portion.

FIG. 17 illustrates an example of a cross-sectional view of a pixel inthe liquid crystal display device according to one embodiment of thepresent invention. A transistor 1401 illustrated in FIG. 17 includes agate layer 1402 formed over an insulating surface, a gate insulatinglayer 1403 over the gate layer 1402, an oxide semiconductor layer 1404that overlaps with the gate layer 1402 with the gate insulating layer1403 placed therebetween, and a conductive film 1405 and a conductivefilm 1406 that are formed to be stacked over the oxide semiconductorlayer 1404 and function as a source layer and a drain layer. Thetransistor 1401 may further include an insulating layer 1407 formed overthe oxide semiconductor layer 1404. The insulating layer 1407 is formedso as to cover the gate layer 1402, the gate insulating layer 1403, theoxide semiconductor layer 1404, the conductive film 1405, and theconductive film 1406.

An insulating layer 1408 is formed over the insulating layer 1407. Anopening is provided in part of the insulating layers 1407 and 1408, anda pixel electrode 1410 is formed so as to be in contact with theconductive film 1406 in the opening.

Further, a spacer 1417 for controlling the cell gap of a liquid crystalelement is formed over the insulating layer 1408. The spacer 1417 can beformed by etching of an insulating film into a desired shape.Alternatively, the cell gap may be controlled by dispersing sphericalspacers over the insulating layer 1408.

An alignment film 1411 is formed over the pixel electrode 1410. Acounter electrode 1413 facing the pixel electrode 1410 is formed on acounter substrate 1420. An alignment film 1414 is formed on a surface ofthe counter electrode 1413, which faces the pixel electrode 1410. Thealignment films 1411 and 1414 can be formed using an organic resin suchas polyimide or polyvinyl alcohol. Alignment treatment such as rubbingis performed on their surfaces in order to align liquid crystalmolecules in a certain direction. Rubbing can be performed in such amanner that a roller wrapped with cloth of nylon or the like is rotatedwhile being in contact with the alignment film, and rubs the surface ofthe alignment film in a certain direction. Note that by using aninorganic material such as silicon oxide, the alignment films 1411 and1414 with alignment characteristics can be directly formed by anevaporation method without performing alignment process.

Furthermore, a liquid crystal 1415 is provided in a region that issurrounded by a sealant 1416 between the pixel electrode 1410 and thecounter electrode 1413. The liquid crystal 1415 may be injected with adispenser method (a dripping method) or a dipping method (a pumpingmethod). Note that a filler may be mixed in the sealant 1416.

A light-blocking film that can block light may be formed between pixelsso that disclination due to alignment disorder of the liquid crystal1415 between the pixels is prevented from being perceived. Thelight-blocking film can be formed using an organic resin containing ablack pigment such as a carbon black or low-order titanium oxide, or afilm containing chromium.

The pixel electrode 1410 and the counter electrode 1413 can be formedusing a transparent conductive material such as indium tin oxidecontaining silicon oxide (ITSO), indium tin oxide (ITO), zinc oxide(ZnO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO), forexample.

Note that a TN (twisted nematic) liquid crystal display device is shownhere; alternatively, the liquid crystal display device may be a VA(vertical alignment) liquid crystal display device, an OCB (opticallycompensated birefringence) liquid crystal display device, an IPS(in-plane switching) liquid crystal display device, or an MVA(multi-domain vertical alignment) liquid crystal display device, forexample.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while the temperature of cholestericliquid crystal is increased. Since the blue phase is only generatedwithin a narrow range of temperature, a chiral agent or an ultravioletcurable resin is added so that the temperature range is increased.Specifically, a liquid crystal composition containing a chiral agent at5 wt % or more is used as the liquid crystal 1415. The liquid crystalcomposition that includes liquid crystal exhibiting a blue phase and achiral agent has such characteristics that the response time is as shortas 10 μs to 100 μs, the alignment process is unnecessary because theliquid crystal composition has optical isotropy, and viewing angledependency is small. The liquid crystal having such features ispreferably used particularly as liquid crystal in the above liquidcrystal display device (the liquid crystal display device in which imagesignals need to be input to each pixel plural times in order to producean image).

Note that although the liquid crystal element in which the liquidcrystal 1415 is sandwiched between the pixel electrode 1410 and thecounter electrode 1413 is shown in FIG. 17 as an example, the liquidcrystal display device according to one embodiment of the presentinvention is not limited to having this structure. A pair of electrodesmay be formed over one substrate as in an IPS liquid crystal element ora liquid crystal element using a blue phase.

(Specific Example of Connection between Pixel Portion and DriverCircuit)

Next, a terminal connection method for directly mounting a substrateprovided with a driver circuit on a substrate provided with a pixelportion will be described.

FIG. 18A is a cross-sectional view of a portion where a substrate 900provided with a driver circuit and a substrate 901 provided with a pixelportion are connected to each other by a wire bonding method. Thesubstrate 900 is attached onto the substrate 901 with an adhesive 903. Atransistor 906 included in the driver circuit is provided on thesubstrate 900. The transistor 906 is electrically connected to a pad 907that is formed to be exposed on a surface of the substrate 900 andfunctions as a terminal. A terminal 904 is formed over the substrate 901in FIG. 18A, and the pad 907 and the terminal 904 are connected to eachother with a wire 905.

FIG. 18B is a cross-sectional view of a portion where a substrate 911provided with a pixel portion and a substrate 910 provided with a drivercircuit are connected to each other by a flip-chip method. In FIG. 18B,a solder ball 913 is connected to a pad 912 that is formed to be exposedon a surface of the substrate 910. Thus, a transistor 914 included inthe driver circuit formed on the substrate 910 is electrically connectedto the solder ball 913 through the pad 912. The solder ball 913 iselectrically connected to a terminal 916 formed over the substrate 911.

The solder ball 913 and the terminal 916 can be connected to each otherby a variety of methods such as thermocompression bonding andthermocompression bonding with ultrasonic vibration. Note that in orderto increase the mechanical strength of the connection portion or thediffusion efficiency of heat generated at the substrate 911, anunderfill may be provided between the substrate 910 and the substrate911 to fill the gap between the solder balls after bonding. Although notnecessarily provided, an underfill can prevent occurrence of aconnection failure due to stress generated by a mismatch in coefficientof terminal expansion between the substrate 910 and the substrate 911.In the case where the solder ball 913 and the terminal 916 are bonded toeach other with ultrasonic waves, connection failures can be reduced ascompared to the case where they are bonded to each other only bythermocompression bonding.

A flip-chip method is suitable to realize connection with a large numberof terminals because even if the number of pads to be connected isincreased, the distance between the pads can be relatively large ascompared to the case of employing a wire bonding method.

Note that the solder ball may be formed by a droplet discharge method bywhich a dispersion liquid in which metal nanoparticles are dispersed isdischarged.

FIG. 18C is a cross-sectional view of a portion where a substrate 921provided with a pixel portion and a substrate 920 provided with a drivercircuit are connected to each other with an anisotropic conductiveresin. In FIG. 18C, a pad 922 that is formed to be exposed on a surfaceof the substrate 920 is electrically connected to a transistor 924included in the driver circuit formed on the substrate 920. The pad 922is connected to a terminal 926 formed over the substrate 921 with ananisotropic conductive resin 927.

Note that the connection method is not limited to the methods shown inFIGS. 18A to 18C. The substrates may be connected to each other with acombination of a wire bonding method and a flip-chip method.

(Specific Example of Driver Circuit mounted on Substrate including PixelPortion)

Next, a mounting method of a substrate including a driver circuit (alsoreferred to as an IC chip) will be described. In the liquid crystaldisplay device according to one embodiment of the present invention, atransistor including a channel formation region formed using an oxidesemiconductor is used, so that a pixel portion and part of drivercircuit can be formed over one substrate.

In a liquid crystal display device illustrated in FIG. 19A, a pixelportion 6002 and scan line driver circuits 6003 are formed over asubstrate 6001. A counter substrate 6006 overlaps the substrate 6001 soas to cover the pixel portion 6002 and the scan line driver circuits6003. Moreover, a substrate 6004 provided with a signal line drivercircuit is directly mounted on the substrate 6001. Specifically, thesignal line driver circuit formed on the substrate 6004 is attached tothe substrate 6001 and electrically connected to the pixel portion 6002.Power supply potentials, various signals, and the like are suppliedthrough an FPC 6005 to the pixel portion 6002, the scan line drivercircuits 6003, and the signal line driver circuit formed on thesubstrate 6004.

In a liquid crystal display panel illustrated in FIG. 19B, a pixelportion 6102 and scan line driver circuits 6103 are formed over asubstrate 6101. A counter substrate 6106 overlaps the substrate 6101 soas to cover the pixel portion 6102 and the scan line driver circuits6103. Moreover, a substrate 6104 provided with a signal line drivercircuit is mounted on a FPC 6105 connected to the substrate 6101. Powersupply potentials, various signals, and the like are supplied throughthe FPC 6105 to the pixel portion 6102, the scan line driver circuits6103, and the signal line driver circuit formed on the substrate 6104.

In a liquid crystal display device illustrated in FIG. 19C, a pixelportion 6202, scan line driver circuits 6203, and a portion 6207 of asignal line driver circuit are formed over a substrate 6201. A countersubstrate 6206 overlaps the substrate 6201 so as to cover the pixelportion 6202, the scan line driver circuits 6203, and the portion 6207of the signal line driver circuit. A substrate 6204 provided withanother portion of the signal line driver circuit are directly mountedon the substrate 6201. Specifically, another portion of the signal linedriver circuit formed on the substrate 6204 is attached to the substrate6201 and electrically connected to the portion 6207 of the signal linedriver circuit. Power supply potentials, various signals, and the likeare supplied through an FPC 6205 to the pixel portion 6202, the scanline driver circuits 6203, the portion 6207 of the signal line drivercircuit, and another portion of the signal line driver circuit formed onthe substrate 6204.

There is no particular limitation on the mounting method of thesubstrate, and a known COG method, wire bonding method, TAB method, orthe like can be used. The position where an IC chip is mounted is notlimited to the positions shown in FIGS. 19A to 19C as long as electricalconnection is achieved. Furthermore, a controller, a CPU, a memory, orthe like may be formed using an IC chip and may be mounted on asubstrate provided with a pixel portion.

(Specific Example of Liquid Crystal Display Device)

Next, the appearance of a panel in the liquid crystal display deviceaccording to one embodiment of the present invention will be describedwith reference to FIGS. 20A and 20B. FIG. 20A is a top view of the panelin which a substrate 4001 and a counter substrate 4006 are bonded toeach other with a sealant 4005. FIG. 20B is a cross-sectional view alongthe dashed line A-A′ in FIG. 20A.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 provided over the substrate 4001. Thecounter substrate 4006 is placed over the pixel portion 4002 and thescan line driver circuit 4004. Thus, the pixel portion 4002 and the scanline driver circuit 4004 are sealed together with a liquid crystal 4007by the substrate 4001, the sealant 4005, and the counter substrate 4006.

In addition, a substrate 4021 where a signal line driver circuit 4003 isformed is mounted on the substrate 4001 in a region other than theregion surrounded by the sealant 4005. FIG. 20B illustrates a transistor4009 included in the signal line driver circuit 4003, as an example.

A plurality of transistors are included in the pixel portion 4002 andthe scan line driver circuit 4004 provided over the substrate 4001. FIG.20B illustrates a transistor 4010 and a transistor 4022 that areincluded in the pixel portion 4002. In each of the transistors 4010 and4022, a channel formation region is formed using an oxide semiconductor.

A pixel electrode 4030 included in a liquid crystal element 4011 iselectrically connected to the transistor 4010. A counter electrode 4031of the liquid crystal element 4011 is formed on the counter substrate4006. The liquid crystal element 4011 corresponds to a region where thepixel electrode 4030, the counter electrode 4031, and the liquid crystal4007 overlap with each other.

A spacer 4035 is provided in order to control a distance (a cell gap)between the pixel electrode 4030 and the counter electrode 4031. FIG.20B shows the case where the spacer 4035 is formed by patterning of aninsulating film; alternatively, a spherical spacer may be used.

A variety of signals and potentials that are applied to the signal linedriver circuit 4003, the scan line driver circuit 4004, and the pixelportion 4002 are supplied from a connection terminal 4016 throughleading wirings 4014 and 4015. The connection terminal 4016 iselectrically connected to a FPC 4018 through an anisotropic conductivefilm 4019.

For the substrate 4001, the counter substrate 4006, and the substrate4021, glass, ceramics, or plastics can be used. Examples of plastics area fiberglass-reinforced plastic (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, and an acrylic resin film.

Note that for a substrate to be placed in the direction from which lighttransmitted through the liquid crystal element 4011 is extracted, alight-transmitting material such as a glass plate, plastics, a polyesterfilm, or an acrylic film is used.

FIG. 21 shows an example of a perspective view illustrating thestructure of the liquid crystal display device according to oneembodiment of the present invention. The liquid crystal display devicein FIG. 21 includes a panel 1601 including a pixel portion, a firstdiffusion plate 1602, a prism sheet 1603, a second diffusion plate 1604,a light guide plate 1605, a backlight panel 1607, a circuit board 1608,and a substrate 1611 provided with a signal line driver circuit.

The panel 1601, the first diffusion plate 1602, the prism sheet 1603,the second diffusion plate 1604, the light guide plate 1605, and thebacklight panel 1607 are stacked in this order. The backlight panel 1607has a backlight 1612 including a plurality of backlight units. Lightthat is emitted from the backlight 1612 and diffused in the light guideplate 1605 is delivered to the panel 1601 through the first diffusionplate 1602, the prism sheet 1603, and the second diffusion plate 1604.

Although the first diffusion plate 1602 and the second diffusion plate1604 are used here, the number of diffusion plates is not limited to twobut may be one, or may be three or more. The diffusion plate should beprovided between the light guide plate 1605 and the panel 1601.Therefore, the diffusion plate may be provided only on the side closerto the panel 1601 than the prism sheet 1603, or may be provided only onthe side closer to the light guide plate 1605 than the prism sheet 1603.

The prism sheet 1603 is not limited to having a sawtooth shape insection as illustrated in FIG. 21 and can have a shape with which lightfrom the light guide plate 1605 can be concentrated on the panel 1601side.

The circuit board 1608 is provided with a circuit that generates variouskinds of signals input to the panel 1601, a circuit that processes thesesignals, and the like. In FIG. 21, the circuit board 1608 and the panel1601 are connected to each other via a COF (chip on film) tape 1609.Moreover, the substrate 1611 provided with the signal line drivercircuit is connected to the COF tape 1609 by a COF method.

FIG. 21 illustrates the example in which the circuit board 1608 isprovided with a controller circuit that controls driving of thebacklight 1612 and the controller circuit and the backlight panel 1607are connected to each other via an FPC 1610. Note that the controllercircuit may be formed in the panel 1601; in that case, the panel 1601and the backlight panel 1607 are to be connected to each other via anFPC or the like.

(Specific Example of Liquid Crystal Display Device Including TouchPanel)

The liquid crystal display device according to one embodiment of thepresent invention may include a pointing device called a touch panel.FIG. 22A illustrates a state where a touch panel 1620 overlaps with apanel 1621.

In the touch panel 1620, a position touched by a finger, a stylus, orthe like is detected in a light-transmitting position detection portion1622 and a signal including information on the position can begenerated. Thus, by providing the touch panel 1620 so that the positiondetection portion 1622 overlaps with a pixel portion 1623 of the panel1621, information on a position in the pixel portion 1623 the user ofthe liquid crystal display device touches can be obtained.

The position can be detected in the position detection portion 1622 by avariety of methods such as a resistive touchscreen technology and acapacitive touchscreen technology. FIG. 22B is a perspective view of theposition detection portion 1622 with a resistive touchscreen technology.In the position detection portion 1622 with a resistive touchscreentechnology, a plurality of first electrodes 1630 and a plurality ofsecond electrodes 1631 are provided so as to face each other with aspace therebetween. When one of the plurality of first electrodes 1630is pressed by the finger or the like, the first electrode 1630 is incontact with one of the plurality of second electrodes 1631. Then, bymonitoring the level of voltage at opposite ends of each of the firstelectrodes 1630 and the level of voltage at opposite ends of each of thesecond electrodes 1631, it is possible to specify which of the firstelectrodes 1630 is in contact with the second electrode 1631; thus, theposition touched by the finger can be detected.

The first electrodes 1630 and the second electrodes 1631 can be formedusing a light-transmitting conductive material, for example, indium tinoxide containing silicon oxide (ITSO), indium tin oxide (ITO), zincoxide (ZnO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO).

FIG. 23A is a perspective view of the position detection portion 1622with a projected capacitive touchscreen technology among capacitivetouchscreen technologies. In the position detection portion 1622 with aprojected capacitive touchscreen technology, a plurality of firstelectrodes 1640 and a plurality of second electrodes 1641 are providedso as to overlap with each other. The first electrodes 1640 each have astructure in which a plurality of rectangular conductive films 1642 areconnected to each other. The second electrodes 1641 each have astructure in which a plurality of rectangular conductive films 1643 areconnected to each other. Note that the shapes of the first electrodes1640 and the second electrodes 1641 are not limited thereto.

In FIG. 23A, an insulating layer 1644 functioning as a dielectricoverlaps the plurality of first electrodes 1640 and the plurality ofsecond electrodes 1641. FIG. 23B illustrates a state where the pluralityof first electrodes 1640, the plurality of second electrodes 1641, andthe insulating layer 1644 illustrated in FIG. 23A overlap with eachother. As illustrated in FIG. 23B, the plurality of first electrodes1640 and the plurality of second electrodes 1641 overlap with each otherso that the position of the rectangular conductive films 1642 does notcorrespond to that of the rectangular conductive films 1643.

When the finger or the like touches the insulating layer 1644,capacitance is generated between one of the plurality of firstelectrodes 1640 and the finger. Moreover, capacitance is also generatedbetween one of the plurality of second electrodes 1641 and the finger.Accordingly, monitoring of the change in capacitance can specify whichfirst electrode 1640 and which second electrode 1641 are closest to thefinger; thus, the position touched by the finger can be detected.

(Example of Method for Manufacturing Transistor)

Next, an example of a method for manufacturing a transistor will bedescribed.

First, as illustrated in FIG. 24A, a gate layer 801 and an electrodelayer 802 are formed over a substrate 800 having an insulating surface.

The gate layer 801 and the electrode layer 802 can be formed with asingle-layer structure or a stacked structure using one or more ofconductive films using a metal material such as molybdenum, titanium,chromium, tantalum, tungsten, neodymium, or scandium or an alloymaterial containing any of these metal materials as a main component, ornitride of these metals. Note that aluminum or copper can also be usedas the metal material if it can withstand the temperature of heattreatment to be performed in a later step. Aluminum or copper ispreferably combined with a refractory metal material so as to preventproblems of low heat resistance and corrosion. As the refractory metalmaterial, molybdenum, titanium, chromium, tantalum, tungsten, neodymium,scandium, or the like can be used.

For example, as a two-layer structure of the gate layer 801 and theelectrode layer 802, the following structures are preferable: atwo-layer structure in which a molybdenum film is stacked over analuminum film, a two-layer structure in which a molybdenum film isstacked over a copper film, a two-layer structure in which a titaniumnitride film or a tantalum nitride film is stacked over a copper film,and a two-layer structure in which a titanium nitride film and amolybdenum film are stacked. As a three-layer structure of the gatelayer 801 and the electrode layer 802, it is preferable to employ astacked structure in which an aluminum film, an alloy film of aluminumand silicon, an alloy film of aluminum and titanium, or an alloy film ofaluminum and neodymium is used as a middle layer and sandwiched betweena top layer and a bottom layer of tungsten, tungsten nitride, titaniumnitride, or titanium.

Further, a light-transmitting oxide conductive film of indium oxide, analloy of indium oxide and tin oxide, an alloy of indium oxide and zincoxide, zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, zincgallium oxide, or the like can be used as the gate layer 801 and theelectrode layer 802.

The thickness of each of the gate layer 801 and the electrode layer 802is 10 nm to 400 nm, preferably 100 nm to 200 nm Here, after a150-nm-thick conductive film for the gate electrode is formed bysputtering using a tungsten target, the conductive film is processed(patterned) into a desired shape by etching, so that the gate layer 801and the electrode layer 802 are formed. The gate layer preferably hastapered edge because coverage of the gate layer with a gate insulatinglayer to be stacked thereover can be improved. Note that a resist maskmay be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing costs can bereduced.

Next, as illustrated in FIG. 24B, a gate insulating layer 803 is formedover the gate layer 801 and the electrode layer 802. The gate insulatinglayer 803 can be formed with a single-layer structure or a stackedstructure of a silicon oxide film, a silicon nitride film, a siliconoxynitride film, a silicon nitride oxide film, an aluminum oxide film,an aluminum nitride film, an aluminum oxynitride film, an aluminumnitride oxide film, a hafnium oxide film, and/or a tantalum oxide filmby plasma CVD, sputtering, or the like. It is preferable that the gateinsulating layer 803 include impurities such as moisture, hydrogen, oroxygen as little as possible. In the case where a silicon oxide film isformed by sputtering, a silicon target or a quartz target is used as atarget, and oxygen or a mixed gas of oxygen and argon is used as asputtering gas.

An oxide semiconductor that becomes intrinsic (i-type) or substantiallyintrinsic by removal of impurities (a purified oxide semiconductor) isquite susceptible to the interface state density or interface charge;therefore, the interface between the purified oxide semiconductor andthe gate insulating layer 803 is important. For that reason, the gateinsulating layer (GI) that is in contact with the purified oxidesemiconductor needs to have higher quality.

For example, high-density plasma CVD using microwaves (e.g., a frequencyof 2.45 GHz) is preferably used because an insulating layer can be denseand have high withstand voltage and high quality. The purified oxidesemiconductor and the high-quality gate insulating layer are in closecontact with each other, so that the interface state density can bereduced to obtain favorable interface characteristics.

Needless to say, another film formation method such as sputtering orplasma CVD can be employed as long as the method enables formation of agood-quality insulating layer as a gate insulating layer. Moreover, itis possible to form an insulating layer whose quality as a gateinsulating layer and characteristics of an interface with the oxidesemiconductor are improved through heat treatment performed after theformation of the insulating layer. In any case, any insulating layer canbe used as long as it can reduce the interface state density between thegate insulating layer and the oxide semiconductor and form a favorableinterface as well as having good film quality as the gate insulatinglayer.

The gate insulating layer 803 may have a structure in which aninsulating layer formed using a material with high barrier propertiesand an insulating layer with a low nitrogen content, such as a siliconoxide film or a silicon oxynitride film, are stacked. In that case, theinsulating layer such as a silicon oxide film or a silicon oxynitridefilm is formed between the insulating layer having high barrierproperties and an oxide semiconductor layer. Examples of the insulatinglayer having high barrier properties are a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, and an aluminumnitride oxide film. With an insulating layer having high barrierproperties, impurities in an atmosphere, such as moisture or hydrogen,or impurities included in the substrate, such as an alkali metal or aheavy metal, can be prevented from entering the oxide semiconductorlayer, the gate insulating layer 803, or the interface between the oxidesemiconductor layer and another insulating layer and the vicinitythereof. In addition, by forming the insulating layer with a lownitrogen content, such as a silicon oxide film or a silicon oxynitridefilm, so as to be in contact with the oxide semiconductor layer, theinsulating layer having high barrier properties can be prevented frombeing in direct contact with the oxide semiconductor layer.

For example, a 100-nm-thick gate insulating layer 803 may be formed asfollows: a silicon nitride film (SiN_(y) (y>0)) with a thickness of 50nm to 200 nm is formed by sputtering as a first gate insulating layer,and a silicon oxide film (SiO_(x) (x>0)) with a thickness 5 nm to 300 nmis stacked over the first gate insulating layer as a second gateinsulating layer. The thickness of the gate insulating layer 803 may beset as appropriate depending on characteristics needed for a transistorand may be approximately 350 nm to 400 nm.

Here, the gate insulating layer 803 in which a 100-nm-thick siliconoxide film formed by sputtering is stacked over a 50-nm-thick siliconnitride film formed by sputtering is formed.

In order for the gate insulating layer 803 to contain hydrogen, ahydroxyl group, and moisture as little as possible, it is preferablethat an impurity adsorbed on the substrate 800, such as moisture orhydrogen, be eliminated and removed by preheating the substrate 800,over which the gate layer 801 and the electrode layer 802 are formed, ina preheating chamber of a sputtering apparatus, as a pretreatment forfilm formation. The temperature for the preheating is 100° C. to 400°C., preferably, 150° C. to 300° C. As an exhaustion unit provided in thepreheating chamber, a cryopump is preferably used. Note that thispreheating treatment can be omitted.

Next, an oxide semiconductor layer having a thickness of 2 nm to 200 nm,preferably 3 nm to 50 nm, further preferably 3 nm to 20 nm is formedover the gate insulating layer 803. The oxide semiconductor layer isformed by sputtering using an oxide semiconductor target. Moreover, theoxide semiconductor layer can be formed by sputtering in a rare gas(e.g., argon) atmosphere, an oxygen atmosphere, or a mixed atmosphereincluding a rare gas (e.g., argon) and oxygen.

Note that before the oxide semiconductor layer is formed by sputtering,dust attached to a surface of the gate insulating layer 803 ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering is a methodby which without application of voltage to a target side, voltage isapplied to a substrate side with an RF power source in an argonatmosphere so that plasma is generated in the vicinity of the substrateto modify a surface. Note that instead of an argon atmosphere, anitrogen atmosphere, a helium atmosphere, or the like may be used.Alternatively, an argon atmosphere to which oxygen, nitrous oxide, orthe like is added; or an argon atmosphere to which chlorine, carbontetrafluoride, or the like is added may be used.

As described above, the oxide semiconductor layer can be formed usingany of the following oxide semiconductors: an In—Sn—Ga—Zn—O-based oxidesemiconductor which is an oxide of four metal elements; anIn—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, and a Sn—Al—Zn—O-based oxide semiconductor which areoxides of three metal elements; an In—Ga—O-based oxide semiconductor, anIn—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor,an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxidesemiconductor, a Sn—Mg—O-based oxide semiconductor, and an In—Mg—O-basedoxide semiconductor which are oxides of two metal elements; and anIn—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, and aZn—O-based oxide semiconductor which are oxides of one metal element.The above oxide semiconductors may include silicon oxide.

As the oxide semiconductor layer, a thin film represented byInMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Gaand Al, Ga and Mn, or Ga and Co.

Here, as the oxide semiconductor layer, a 30-nm-thick In—Ga—Zn—O-basednon-single-crystal film obtained by sputtering using a metal oxidetarget containing indium (In), gallium (Ga), and zinc (Zn) is used. Asthe target, a metal oxide target having a composition ratio ofIn:Ga:Zn=1:1:0.5, In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used, forexample. The target may contain SiO₂ at 2 wt % to 10 wt %. The fillingrate of the metal oxide target containing In, Ga, and Zn is 90% to 100%,preferably 95% to 100%. By using the metal oxide target with a highfilling rate, a dense oxide semiconductor layer is formed.

Here, the oxide semiconductor layer is formed over the substrate 800 insuch a manner that the substrate is held in the treatment chamber keptat reduced pressure, a sputtering gas from which hydrogen and moisturehave been removed is introduced into the treatment chamber whilemoisture remaining therein is removed, and the above-described target isused. The substrate temperature may be 100° C. to 600° C., preferably200° C. to 400° C. in film formation. By heating the substrate duringfilm formation, the impurity concentration in the oxide semiconductorlayer formed can be reduced. In addition, damage by sputtering can bereduced. In order to remove remaining moisture in the treatment chamber,an entrapment vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thetreatment chamber which is evacuated with the cryopump, for example, ahydrogen atom, a compound containing a hydrogen atom, such as water(H₂O) (more preferably, also a compound containing a carbon atom), andthe like are removed, whereby the impurity concentration in the oxidesemiconductor layer formed in the treatment chamber can be reduced.

As one example of the deposition conditions, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power source is 0.5 kW, and the atmosphere is anoxygen atmosphere (the proportion of the oxygen flow rate is 100%). Notethat a pulsed direct-current (DC) power source is preferable becausepowder substances (referred to as particles) generated in filmdeposition can be reduced and the film thickness can be uniform.

In order that the oxide semiconductor layer does not to containimpurities such as hydrogen, a hydroxyl group, or moisture as little aspossible, it is preferable to preheat the substrate 800 provided withthe gate insulating layer 803 in a preheating chamber of the sputteringapparatus before the film formation so that impurities such as moistureor hydrogen adsorbed on the substrate 800 is eliminated and removed. Thetemperature for the preheating is 100° C. to 400° C., preferably 150° C.to 300° C. As an exhaustion unit provided in the preheating chamber, acryopump is preferably used. Note that this preheating treatment can beomitted. In addition, before an insulating layer 808 is formed, thepreheating may similarly be performed on the substrate 800 over whichlayers up to and including a source layer 805, a drain layer 806, and anelectrode layer 807 are formed.

Next, as illustrated in FIG. 24B, the oxide semiconductor layer isprocessed (patterned) into a desired shape by etching or the like, sothat an island-shaped oxide semiconductor layer 804 is formed over thegate insulating layer 803 to overlap with the gate layer 801.

A resist mask for forming the island-shaped semiconductor layer 804 maybe formed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, manufacturing costs can be reduced.

Note that etching for forming the island-shaped oxide semiconductorlayer 804 may be wet etching, dry etching, or both dry etching and wetetching. As an etching gas used for dry etching, a gas containingchlorine (a chlorine-based gas such as chlorine (Cl₂), boron trichloride(BCl₃), silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄))is preferably used. Alternatively, a gas containing fluorine (afluorine-based gas such as carbon tetrafluoride (CF₄), sulfurhexafluoride (SF₆), nitrogen trifluoride (NF₃), or trifluoromethane(CHF₃)), hydrogen bromide (HBr), oxygen (O₂), any of these gases towhich a rare gas such as helium (He) or argon (Ar) is added, or the likecan be used.

For dry etching, a parallel plate RIE (reactive ion etching) method oran ICP (inductively coupled plasma) etching method can be used. In orderto etch the films into desired shapes, the etching conditions (theamount of power applied to a coil-shaped electrode, the amount of powerapplied to an electrode on the substrate side, and the temperature ofthe electrode on the substrate side) are adjusted as appropriate.

As an etchant used for wet etching, ITO-07N (produced by Kanto ChemicalCo., Inc.) is used. After the wet etching, the etchant is removedtogether with the etched materials by cleaning. The waste liquidincluding the etchant and the material etched off may be purified andthe material may be reused. When a material such as indium contained inthe oxide semiconductor layer is collected from the waste liquid afterthe etching and reused, the resources can be efficiently used and thecosts can be reduced.

Note that it is preferable that reverse sputtering be performed before aconductive film is formed in a subsequent step, in order to remove aresist residue or the like attached to surfaces of the island-shapedoxide semiconductor layer 804 and the gate insulating layer 803.

Then, heat treatment is performed on the oxide semiconductor layer 804in a nitrogen atmosphere, an oxygen atmosphere, an atmosphere ofultra-dry air (air in which the water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (e.g., argon or helium) atmosphere. Heat treatment performed on theoxide semiconductor layer 804 can eliminate moisture or hydrogen in theoxide semiconductor layer 804. Specifically, heat treatment may beperformed at 350° C. to 850° C. (or the strain point of a glasssubstrate), preferably 550° C. to 750° C. For example, heat treatmentcan be performed at 600° C. for about 3 to 6 minutes. Since dehydrationor dehydrogenation can be performed in a short time with an RTA method,heat treatment can be performed even at a temperature over the strainpoint of a glass substrate. Alternatively, heat treatment may beperformed for about one hour in a state where the substrate temperaturereaches 450° C.

Here, the oxide semiconductor layer 804 is subjected to the heattreatment in a nitrogen atmosphere with the use of an electric furnacewhich is one example of a heat treatment apparatus.

Note that a heat treatment apparatus is not limited to an electricalfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermal anneal)apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA(lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus isan apparatus for heating an object by radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object by heat treatment, suchas nitrogen or a rare gas (e.g., argon) is used.

For example, as the heat treatment, GRTA in which the substrate is movedinto an inert gas heated at a high temperature of 650° C. to 700° C.,heated for several minutes, and moved out of the inert gas heated to thehigh temperature may be performed. With GRTA, high-temperature heattreatment in a short period of time can be achieved.

Note that it is preferable that in the heat treatment, moisture,hydrogen, or the like be not contained in nitrogen or a rare gas such ashelium, neon, or argon. It is preferable that the purity of nitrogen orthe rare gas such as helium, neon, or argon which is introduced into aheat treatment apparatus be set to be 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (i.e., the impurity concentration is1 ppm or lower, preferably 0.1 ppm or lower).

When impurities such as moisture or hydrogen is added to an oxidesemiconductor, in a gate bias-temperature stress test (BT test, the testcondition is, for example, at 85° C. with 2×10⁶ V/cm for 12 hours), abond between the impurities and a main component of the oxidesemiconductor is broken by a high electric field (B: bias) and hightemperature (T: temperature), and a dangling bond generated causes shiftof the threshold voltage (Vth). However, in the above manner,characteristics of the interface between the gate insulating layer andthe oxide semiconductor layer are improved and impurities in the oxidesemiconductor layer, particularly moisture and hydrogen, are removed asmuch as possible; thus, a transistor that can be stable even in a BTtest can be obtained.

Through the above process, the concentration of hydrogen in the oxidesemiconductor layer 804 can be reduced and the oxide semiconductor layercan be purified. Accordingly, the oxide semiconductor layer can bestable. In addition, heat treatment at a temperature which is lower thanor equal to the glass transition temperature makes it possible to forman oxide semiconductor layer with extremely low carrier density and awide band gap. Therefore, a transistor can be manufactured using a largesubstrate, so that the productivity can be increased. Moreover, by usingthe purified oxide semiconductor layer with a reduced hydrogenconcentration, it is possible to form a transistor with high withstandvoltage, reduced short-channel effect, and a high on/off ratio.

Note that in the case where the oxide semiconductor layer is heated,depending on a material or heating conditions of the oxide semiconductorlayer, plate-shaped crystals are sometimes formed on the top surface ofthe oxide semiconductor layer. The plate-shaped crystals are preferablysingle crystals that are oriented in the c-axis (the direction generallyperpendicular to the surface of the oxide semiconductor layer). If theplate-shaped crystals are not single crystals, they are preferablypolycrystals where a-b planes of the crystals are aligned or the a-axesor the b-axes are aligned in the channel formation region and thecrystals are oriented in the c-axis (the direction substantiallyperpendicular to the surface of the oxide semiconductor layer). Notethat in the case where a surface of the layer placed below the oxidesemiconductor layer is uneven, the plate-shaped crystals arepolycrystals; therefore, the surface of the layer placed below the oxidesemiconductor layer is preferably as even as possible.

Next, a conductive film used for the source layer and the drain layer(including a wiring formed using the same layer as the source and drainlayers) is formed over the oxide semiconductor layer 804 by sputteringor vacuum evaporation. Then, the conductive film is patterned by etchingor the like, thereby forming the source layer 805 and the drain layer806 over the oxide semiconductor layer 804, and the wiring layer 807that overlaps with the electrode layer 802 with the gate insulatinglayer 803 placed therebetween, as illustrated in FIG. 24C.

Examples of the material for the conductive film which serves as thesource layer 805, the drain layer 806, and the electrode layer 807(including the wiring formed using the same layer as those layers) arean element selected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloyincluding any of the above elements as a component; and an alloyincluding any of these elements in combination. The conductive film mayhave a structure in which a refractory metal film of Cr, Ta, Ti, Mo, W,or the like is stacked on one or both of a top surface and a bottomsurface of a metal layer of Al, Cu, or the like. Furthermore, the heatresistance can be increased by using an aluminum material to which anelement that prevents generation of hillocks and whiskers in an aluminumfilm, such as Si, Ti, Ta, W, Mo, Cr, Nd, Sc, or Y, is added.

Further, the conductive film may have a single-layer structure or astacked structure of two or more layers. For example, the conductivefilm can have a single-layer structure of an aluminum film containingsilicon; a two-layer structure in which a titanium film is stacked overan aluminum film; or a three-layer structure in which a titanium film,an aluminum film, and a titanium film are stacked in this order.

The conductive film which serves as the source layer 805, the drainlayer 806, and the electrode layer 807 (including the wiring layerformed using the same layer as these layers) may be formed using aconductive metal oxide. As the conductive metal oxide, indium oxide(In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), an alloy of indium oxideand tin oxide (In₂O₃—SnO₂, referred to as ITO), an alloy of indium oxideand zinc oxide (In₂O₃—ZnO), or any of the metal oxide materialscontaining silicon or silicon oxide can be used.

In the case where heat treatment is performed after formation of theconductive film, the conductive film preferably has heat resistanceenough to withstand the heat treatment.

Note that materials and etching conditions are adjusted as appropriateso that the oxide semiconductor layer 804 is not removed in etching ofthe conductive film as much as possible. Depending on the etchingconditions, a groove (a recessed portion) is sometimes formed by etchingof part of an exposed portion of the island-shaped oxide semiconductorlayer 804.

In order to reduce the number of photomasks and steps in aphotolithography step, etching may be performed with the use of a resistmask formed using a multi-tone mask which is a light-exposure maskthrough which light is transmitted to have a plurality of intensities. Aresist mask formed using a multi-tone mask has a plurality ofthicknesses and further can be changed in shape by etching; therefore,the resist mask can be used in a plurality of etching steps forprocessing into different patterns. Therefore, a resist maskcorresponding to at least two kinds of different patterns can be formedby one multi-tone mask. Thus, the number of light-exposure masks can bereduced and the number of corresponding photolithography steps can bealso reduced, so that the process can be simplified.

Next, plasma treatment is performed using a gas such as N₂O, N₂, or Ar.By this plasma treatment, absorbed water and the like attached to anexposed surface of the oxide semiconductor layer are removed.Alternatively, plasma treatment may be performed using a mixture gas ofoxygen and argon.

After the plasma treatment, as illustrated in FIG. 24D, an insulatinglayer 808 is formed so as to cover the source layer 805, the drain layer806, the electrode layer 807, and the oxide semiconductor layer 804. Theinsulating layer 808 preferably contains impurities such as moisture,hydrogen, and oxygen as little as possible, and may be formed using asingle insulating layer or a stack of a plurality of insulating layers.If hydrogen is contained in the insulating layer 808, hydrogen mightenter the oxide semiconductor layer or extract oxygen from the oxidesemiconductor layer, thereby causing reduction in resistance of a backchannel portion of the oxide semiconductor layer (making the backchannel portion have n-type conductivity), which might result information of parasitic channel. Therefore, it is important that theinsulating layer 808 be formed by a method that does not use hydrogen sothat the insulating layer 808 contains as little hydrogen as possible.The insulating layer 808 is preferably formed using a material with highbarrier properties. For example, as an insulating film with high barrierproperties, the insulating film having a high barrier property, asilicon nitride film, a silicon nitride oxide film, an aluminum nitridefilm, an aluminum nitride oxide film, or the like can be used. When aplurality of insulating films stacked is used, an insulating layer witha low content nitrogen, such as a silicon oxide film or a siliconoxynitride film, is formed on the side closer to the oxide semiconductorlayer 804 than the insulating layer with high barrier properties. Then,the insulating layer with high barrier properties is formed so as tooverlap with the source layer 805, the drain layer 806, and the oxidesemiconductor layer 804 with the insulating layer with a low content ofnitrogen placed therebetween. By using the insulating layer with highbarrier properties, impurities such as moisture or hydrogen can beprevented from entering the oxide semiconductor layer 804, the gateinsulating layer 803, or the interface between the oxide semiconductorlayer 804 and another insulating layer and the vicinity thereof. Inaddition, when the insulating layer with a low content of nitrogen, suchas a silicon oxide film or a silicon oxynitride film, is formed incontact with the oxide semiconductor layer 804, the insulating layerformed using a material with high barrier properties can be preventedfrom being directly in contact with the oxide semiconductor layer 804.

Here, the gate insulating layer 808 in which a 100-nm-thick siliconnitride film formed by sputtering is stacked over a 200-nm-thick siliconoxide film formed by sputtering is formed. The substrate temperature atthe time of deposition can be from room temperature to 300° C. or lowerand is 100° C. in this embodiment.

Note that heat treatment may be performed after the insulating layer 808is formed. The heat treatment is performed at preferably 200° C. to 400°C. (e.g., 250° C. to 350° C.) in a nitrogen atmosphere, an oxygenatmosphere, an atmosphere of ultra-dry air (air in which the watercontent is 20 ppm or less, preferably 1 ppm or less, further preferably10 ppb or less), or a rare gas (e.g., argon or helium) atmosphere. Here,for example, heat treatment is performed at 250° C. for 1 hour in anitrogen atmosphere. Alternatively, before the source layer 805, thedrain layer 806, and the electrode layer 807 are formed, RTA treatmentat high temperatures for a short time may be performed in a mannersimilar to the previous heat treatment performed on the oxidesemiconductor layer. Even if oxygen vacancy is created in the oxidesemiconductor layer 804 because of the heat treatment performed on theoxide semiconductor layer, oxygen is supplied to the oxide semiconductorlayer 804 when heat treatment is performed after the insulating layer808 containing oxygen is provided in contact with an exposed region ofthe oxide semiconductor layer 804 placed between the source layer 805and the drain layer 806. Accordingly, by supplying oxygen to the regionof the oxide semiconductor layer 804 which is in contact with theinsulating layer 808, oxygen vacancies serving as donors can be reducedand the stoichiometric composition ratio can be satisfied. As a result,the oxide semiconductor layer 804 can be made to be an intrinsicsemiconductor layer or a substantially intrinsic semiconductor layer. Asa result, an oxide semiconductor film can be made to be an intrinsicsemiconductor film or a substantially intrinsic semiconductor film.Accordingly, electric characteristics of the transistor can be improvedand variation in the electric characteristics thereof can be reduced.The timing of this heat treatment is not particularly limited as long asit is after the formation of the insulating layer 808. When this heattreatment also serves as heat treatment in another step (e.g., heattreatment at the time of formation of a resin film or heat treatment forreducing the resistance of a transparent conductive film), the oxidesemiconductor layer 804 can be intrinsic or substantially intrinsicwithout an increase in the number of steps.

Next, a conductive film may be formed over the insulating layer 808 andpatterned so that a back gate layer may be formed so as to overlap withthe oxide semiconductor layer 804. In the case where the back gate layeris formed, an insulating layer is formed so as to cover the back gatelayer. The back gate layer can be formed using a material and astructure which are similar to those of the gate layer 801 and theelectrode layer 802 or those of the source layer 805, the drain layer806, and the electrode layer 807.

The thickness of the back gate layer is 10 nm to 400 nm, preferably 100nm to 200 nm Here, the back gate layer is formed in the followingmanner: a conductive film in which a titanium film, an aluminum film,and a titanium film are stacked is formed, a resist mask is formed byphotolithography or the like, and unnecessary portions are removed byetching so that the conductive film is processed (patterned) to adesired shape.

The insulating layer is preferably formed using a material with highbarrier properties that can prevent moisture, hydrogen, oxygen, and thelike in an atmosphere from adversely affecting the characteristics ofthe transistor. For example, the insulating layer with high barrierproperties can be formed with a single-layer structure or a stackedstructure of a silicon nitride film, a silicon nitride oxide film, analuminum nitride film, an aluminum nitride oxide film, and/or the likeby plasma CVD, sputtering, or the like. In order to obtain an effect ofbarrier properties, the insulating layer is preferably formed to athickness of 15 nm to 400 nm, for example.

Here, a 300-nm-thick insulating layer is formed by plasma CVD. Thedeposition conditions for the insulating layer are as follows: the flowrate of a silane gas is 4 sccm; the flow rate of dinitrogen monoxide(N₂O) is 800 sccm; and the substrate temperature is 400° C.

Through the above steps, a transistor 809 and a capacitor 810 areformed. Note that the capacitor 810 is formed in a region where theelectrode layer 807 overlaps with the electrode layer 802 with the gateinsulating layer 803 placed therebetween.

The transistor 809 includes the gate layer 801, the gate insulatinglayer 803 over the gate layer 801, the oxide semiconductor layer 804that overlaps with the gate layer 801 with the gate insulating layer 803placed therebetween, and the source layer 805 and the drain layer 806that are formed over the oxide semiconductor layer 804. The transistor809 may further include the insulating layer 808 provided over the oxidesemiconductor layer 804 as its component. The transistor 809 illustratedin FIG. 24D has a channel-etched structure in which part of the oxidesemiconductor layer 804 between the source layer 805 and the drain layer806 is etched.

Note that the transistor 809 is described as a single-gate transistor;alternatively, a multi-gate transistor including a plurality of channelformation regions by including a plurality of the gate layers 801 thatare electrically connected to each other can be manufactured if needed.

(Various Electronic Devices Including Liquid Crystal Display Device)

Examples of electronic devices including any of the liquid crystaldisplay devices disclosed in this specification will be described belowwith reference to FIGS. 25A to 25F.

FIG. 25A illustrates a notebook personal computer including a main body2201, a housing 2202, a display portion 2203, a keyboard 2204, and thelike.

FIG. 25B illustrates a personal digital assistant (PDA). A main body2211 is provided with a display portion 2213, an external interface2215, operation buttons 2214, and the like. A stylus 2212 is provided asan accessory for operating the PDA.

FIG. 25C illustrates an e-book reader 2220 as an example of electronicpaper. The e-book reader 2220 includes two housings of a housing 2221and a housing 2223. The housings 2221 and 2223 are united with an axisportion 2237, along which the e-book reader 2220 can be opened andclosed. With such a structure, the e-book reader 2220 can be used like apaper book.

A display portion 2225 is incorporated in the housing 2221, and adisplay portion 2227 is incorporated in the housing 2223. The displayportion 2225 and the display portion 2227 may display one image ordifferent images. In the case where the display portions 2225 and 2227display different images, for example, the right display portion (thedisplay portion 2225 in FIG. 25C) can display text and the left displayportion (the display portion 2227 in FIG. 25C) can display pictures.

Further, in FIG. 25C, the housing 2221 is provided with an operationportion and the like. For example, the housing 2221 is provided with apower switch 2231, an operation key 2233, and a speaker 2235. Pages canbe turned with the operation key 2233. Note that a keyboard, a pointingdevice, or the like may also be provided on the surface of the housing,on which the display portion is provided. An external connectionterminal (e.g., an earphone terminal, a USB terminal, or a terminal thatcan be connected to an AC adapter or various cables such as a USBcable), a recording medium insertion portion, and the like may beprovided on the back surface or the side surface of the housing.Further, the e-book reader 2220 may have a function of an electronicdictionary.

The e-book reader 2220 may be configured to transmit and receive datawirelessly. Through wireless communication, desired book data or thelike can be purchased and downloaded from an e-book server.

Note that electronic paper can be applied to devices in a variety offields as long as they display data. For example, electronic paper canbe used for posters, advertisement in vehicles such as trains, anddisplay in a variety of cards such as credit cards in addition to e-bookreaders.

FIG. 25D illustrates a mobile phone. The mobile phone includes twohousings of a housing 2240 and a housing 2241. The housing 2241 isprovided with a display panel 2242, a speaker 2243, a microphone 2244, apointing device 2246, a camera lens 2247, an external connectionterminal 2248, and the like. The housing 2240 is provided with a solarcell 2249 for charging the mobile phone, an external memory slot 2250,and the like. An antenna is incorporated in the housing 2241.

The display panel 2242 has a touch panel function. In FIG. 25D, aplurality of operation keys 2245 displayed as images are shown by dashedlines. Note that the mobile phone includes a booster circuit forincreasing a voltage output from the solar cell 2249 to a voltage neededfor each circuit. Moreover, the mobile phone can include a contactlessIC chip, a small recording device, or the like in addition to the abovecomponents.

The display orientation of the display panel 2242 changes as appropriatein accordance with the application mode. Further, the camera lens 2247is provided on the same surface as the display panel 2242, so that themobile phone can be used as a video phone. The speaker 2243 and themicrophone 2244 can be used for videophone calls, recording, playingsound, and the like as well as voice calls. The housings 2240 and 2241which are unfolded as illustrated in FIG. 25D can slide so that oneoverlaps the other. Thus, the size of the mobile phone can be reduced,which makes the mobile phone suitable for being carried.

The external connection terminal 2248 can be connected to an AC adapteror a variety of cables such as a USB cable, which enables charging ofthe mobile phone and data communication. Moreover, a larger amount ofdata can be saved and moved by inserting a recording medium to theexternal memory slot 2250. Further, the mobile phone may have aninfrared communication function, a television reception function, or thelike in addition to the above functions.

FIG. 25E illustrates a digital camera. The digital camera includes amain body 2261, a display portion (A) 2267, an eyepiece 2263, anoperation switch 2264, a display portion (B) 2265, a battery 2266, andthe like.

FIG. 25F illustrates a television set. In a television set 2270, adisplay portion 2273 is incorporated in a housing 2271. The displayportion 2273 can display images. Here, the housing 2271 is supported bya stand 2275.

The television set 2270 can be operated by an operation switch of thehousing 2271 or a separate remote controller 2280. With operation keys2279 of the remote controller 2280, channels and volume can becontrolled and an image displayed on the display portion 2273 can becontrolled. Moreover, the remote controller 2280 may have a displayportion 2277 that displays data output from the remote controller 2280.

Note that the television set 2270 is preferably provided with areceiver, a modem, and the like. A general television broadcast can bereceived with the receiver. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) data communication can beperformed.

This application is based on Japanese Patent Application serial No.2010-090935 and 2010-114435 filed with Japan Patent Office on Apr. 9,2010 and May 18, 2010, respectively, the entire contents of which arehereby incorporated by reference.

1. (canceled)
 2. A driving method of a liquid crystal display device inwhich each of a plurality of pixels comprises a first transistor, asecond transistor, a third transistor, and a liquid crystal element, thedriving method comprising the steps of: turning on the first transistorin each of the plurality of pixels in first to n-th rows so that a firstimage signal is input to the liquid crystal element through the firsttransistor by sequentially inputting a first selection signal to a firstscan line to an n-th scan line in a period, turning on the secondtransistor in each of the plurality of pixels in (n+1)th to 2n-th rowsso that a second image signal is input to the liquid crystal elementthrough the second transistor by sequentially inputting a secondselection signal to a (n+1)th scan line to a 2n-th scan line in aperiod, turning on the third transistor in each of the plurality ofpixels in (2n+1)th to 3n-th rows so that a third image signal is inputto the liquid crystal element through the first transistor bysequentially inputting a third selection signal to a (2n+1)th scan lineto a 3n-th scan line in a period, after finishing inputting the first tothird selection signals respectively from the k-th scan line, from the(n+k)th scan line and from the (2n+k)th scan line, emitting light from alight source for the first to k-th rows, a light source for the (n+1)thto (n+k)th rows and a light source for the (2n+1)th to (2n+k)th rows,wherein n is a natural number of 3 or more, wherein k is a naturalnumber of 2 or more and less than n, and wherein a color of the lightemitted from the light source for the first to k-th rows, a color of thelight emitted from the light source for the (n+1)th to (n+k)th rows, anda color of the light emitted from the light source for the (2n+1)th to(2n+k)th rows are different from one another.
 3. The method for drivinga display device according to claim 2, wherein each of the firsttransistor, the second transistor, and the third transistor comprises anoxide semiconductor layer including a channel formation region, whereinthe oxide semiconductor layer has crystallinity, and wherein the oxidesemiconductor layer is a non-single-crystal film.
 4. The display deviceaccording to claim 2, wherein a carrier concentration of the oxidesemiconductor layer is less than 1×10¹²/cm³.
 5. The display deviceaccording to claim 2, wherein the oxide semiconductor layer includes anIn—Ga—Zn—O-based oxide semiconductor.
 6. The display device according toclaim 2, wherein the oxide semiconductor layer includes anIn—Sn—Zn—O-based oxide semiconductor.
 7. A driving method of a liquidcrystal display device in which each of a plurality of pixels comprisesa first transistor, a second transistor, a third transistor, and aliquid crystal element, the driving method comprising the steps of:turning on the first transistor in each of the plurality of pixels infirst to n-th rows so that a first image signal is input to the liquidcrystal element through the first transistor by sequentially inputting afirst selection signal to a first scan line to an n-th scan line in aperiod, turning on the second transistor in each of the plurality ofpixels in (n+1)th to 2n-th rows so that a second image signal is inputto the liquid crystal element through the second transistor bysequentially inputting a second selection signal to a (n+1)th scan lineto a 2n-th scan line in a period, turning on the third transistor ineach of the plurality of pixels in (2n+1)th to 3n-th rows so that athird image signal is input to the liquid crystal element through thefirst transistor by sequentially inputting a third selection signal to a(2n+1)th scan line to a 3n-th scan line in a period, after finishinginputting the first to third selection signals respectively from thek-th scan line, from the (n+k)th scan line and from the (2n+k)th scanline, emitting light from a light source for the first to k-th rows, alight source for the (n+1)th to (n+k)th rows and a light source for the(2n+1)th to (2n+k)th rows, wherein n is a natural number of 3 or more,wherein k is a natural number of 2 or more and less than n, wherein acolor of the light emitted from the light source for the first to k-throws, a color of the light emitted from the light source for the (n+1)thto (n+k)th rows, and a color of the light emitted from the light sourcefor the (2n+1)th to (2n+k)th rows are different from one another,wherein each of the first transistor, the second transistor, and thethird transistor comprises an oxide semiconductor layer including achannel formation region, wherein the oxide semiconductor layer hascrystallinity, wherein the oxide semiconductor layer is anon-single-crystal film, and wherein the oxide semiconductor layerincludes an In—Ga—Zn—O-based oxide semiconductor.
 8. The display deviceaccording to claim 7, wherein a carrier concentration of the oxidesemiconductor layer is less than 1×10¹²/cm³.